What's good, whippers and snappers. Emo here, with my Russian friend here. Joined the League a pretty long time ago, had a few things to do with the League beforehand, and now has joined to inact his own... something, I bet. He's awesome.
Vik

Viktor, the Machine Herald

Vik
Riot stated Viktor was a "mage, but the style of mage is up to the player". Horse sh¡t. Viktor is both a really strong burst mage and a powerful sustaining mage. Not in the sense of permanently upkeeping max HP-sustaining, but he's really more than capable of holding his own.
Vik
In short - similar to Ciderhelm's "Mini's" - Viktor is a medium range ability power caster. He has a single target nuke that provides a temporary shield. He has a slowing AoE that stuns after a delay. He has a vector skillshot with a possible burn effect. He has an AoE short duration silence, and a redirectable continuous AoE damaging ultimate.
Vik
All in all, Viktor is a really strong champion. I've tried him multiple times before this free week - I got him since his skin bundle, so I know what I'm talking about - and I can honestly say that there's few matchups that he can't take. It takes a good enemy to take on a Viktor.
Vik
~Emo

You start the game with a Hex Core in your inventory. Within the in-game item shop, you can upgrade this Hex Core into one of three "Augments", each enhancing one of Viktor's basic abilities. This costs 1000g and is not reversable.
Viktor
Choose carefully.
Viktor Viktor blasts a single target for magic damage. The projectile then returns to Viktor, shielding him for 40% of the tooltip-stated damage upon arrival. This shield lasts for 3 seconds.
Viktor
The way I play Viktor, it's your best poking tool. Even though it doesn't have the best ratio, range or base damage, it becomes a really strong zoning spell when you purchase a Fiendish Codex, with its cooldown around 4 seconds. It becomes so powerful, if you rank it first, you'll be able to take a quarter of the enemy's health with ease. Take first at Lv.1, max at Lv.9.
Viktor Viktor summons a gravital entrapment device at target location, which slows enemy units in it after a 0,25 second delay. If any enemy remains in it for 1,5 seconds, they are stunned for 1,5 seconds.
Viktor
It's not the best ability around, but the way they buffed it as soon as they noticed it was hard to put to good use has made it a really strong form of CC. Any champion with a reliable gap closer will either face its stun or burn Flash trying to get out of it. When you hit Lv.13, if they don't Flash, it might spell a lot of pain for them. However, it's CC and it deals no damage. Obviously, it becomes our last choice. Taken at Lv.4 for some CC in a rotation, but maxed last.
Viktor Viktor fires a line vector skillshot that deals magic damage to all targets it hits.
Viktor
The simplest of Viktor's abilities, but its simplicity and damage are countered by rediculously high cooldowns and mana costs, which is a real shame. It is, however, a really strong AoE damaging and farming tool, which makes him a rediculously strong farmer post-Rabadon. As this is a damaging ability, but one with a too high mana cost, I take it first at Lv.2 most of the time, and max it by Lv.13.
Viktor Viktor summons a powerful chaos storm at target location, dealing a splash of magic damage and briefly (0,5s) silencing enemy units in the area. After the initial cast, the ability can be re-cast indefinately for up to 7 seconds, redirecting the storm to wherever the cursor is. The storm keeps damaging enemy units each second for the 7 seconds it remains active after its initial cast.
Viktor
This is a really strong ultimate. It provides a nice instant burst damage, which can both be good as a finisher-offer, or as a skill in the rotation when you're certain you want him dead. Additionally, the 'churning' gives it a rediculously high AP Ratio, paired with enormous base figures if a single enemy eats all the damage. 500 AP turns into 1150 bonus damage on it alone, not taking multiple Power Transfers and Hextech Rays into account. As with all good ultis, take whenever you can.

Viktor
There's something with this Hex Core. Or actually, more than something. There are some things about it.
Viktor
The Hex Core automatically appears in your inventory upon starting the game, and with its inability to sell it - ever - within the game, you'll have one less item you're able to buy. That's a big setback. The upgrades it provides do not count up to the 1000 gold, meaning it's not worth Augmenting until you finish your Rabadon's, pretty much. Wasting 1000 gold on an item with such meager stats is a really big setback.
ViktorViktor
Augment: Power enhances your Power Transfer. It now gives an additional 220HP and 6HP/5. It enhances Power Transfer in such a way that you gain a 3 second movespeed boost as soon as the projectile launches. This gives Viktor some mobility, because let's face it: he ain't fast. It makes sticking to a single target a bit easier, especially before you get a Rylai's, and makes follow-ups a lot easier.
Augment: Gravity enhances your Gravity Field. It now gives an additional 10% CDR, 5MP/5 and 200 Mana. I enhances Gravity Field with 30% additional casting range. This is the Augment I usually buy. Seeing as how I rank up Gravity Field last, it needs to keep up with me, and with its range literally being half a minion more than Power Transfer, the increased range is a-fµcking-mazing during team fights and turret stand-offs.
Augment: Death enhances your Hextech Ray. It now simply gives an additional 45 Flat Ability Power. It enhances Hextech Ray with a 30% damage burn effect over 4 seconds. It's the most commonly used Augment, simply because it adds damage, and gives some additional damage to boost this added damage some more. However, its entire existence lacks any utility, so I really never take it outside botgames.

Runes

Viktor
Runes are a standard page for me. I go with Flat Magic Pen. Marks for some added early game damage; Flat Mana Regeneration Seals because I feel it's important to be able to keep up the damage with Power Transfer, and any and all mana regeneration helps; Scaling Ability Power Glyphs because they're leveled out with Flats as soon as Lv.6 I believe, making it a better choice overall; and Flat Ability Power Quints, just for that strong early game start. You'll lack a lot of damage if you'd start with Scaling Quints, believe you me.Viktor

Masteries

Viktor
I've transitioned from the standard 21/0/9 into a 9/0/21 build. Some might agree, most won't. It's as simple as that. I just really like how I can hit the cooldown cap with this Mastery Setup, a Fiendish Codex and the Crest of the Ancient Golem, putting out a Power Transfer permanently. I take all Ability Power focused masteries in Offense, and taking Increased Neutral Buff Duration, MoveSpeed, Starting Gold and Cooldown Reduction in Utility. It gives me an extra Mana Potion which is nice, some added MS (duh), and 30s extra uptime on my Crest. Good Stuffs.
Viktor

Summoner Spells

My summoner spells are as follows:
Viktor

space

Best choice on any mage. The Summoner's Wrath mastery gives 5AP and 5AD when we use it, which is nice, and sometimes, your damage just isn't enough to take all of the enemies' health. For that - and to counter Dr. Mundo - we have Ignite.

Viktor

space

Besides Gravity Field, Viktor has no escape mechs. A true warrior would take Ignite + Exhaust or some sh*t, but I'm no warrior. I'm a techmaturgically adapted scientist, so I take Flash.

So you'll probably have seen the Fiendish Codex in the item quickguide up top. Might be an odd choice. Explanation is readable up top, I'll put a download link in somewhere later. For those select few lazy motherf*ckers: Codex + Blue + Gravity/Masteries = 40%.
Viktor
Anyways. A quick overview of the items I like to get on Viktor.
Viktor

spa

Only starting item you should get. It allows for a few potions, which in turn mean 300 more effective health, and two more casts on Power Transfer, which are amazing, obviously. Additionally, the movement is better than you eating most AoE damage because you can't outrun it.

Viktor

spa

If your jungler isn't rætarded - like my last Viktor-game's jungler was - you'll get second blue to go with your Fiendish Codex. This means 40% CDR, which means unlimited poking with Power Transfer into Hextech Ray. Enemies will either go B a number of times, or face the horrendous consequences.

Viktor

spa

Best boots for mages. It makes your Power Transfer and Hextech Ray deal true damage (almost, that is), which is no joke, seeing as how they're both on a relatively low cooldown for their damage.

Viktor

spa

Best mage item hands-down. You'll want to get this one fast, since it'll lift you to about 380~ Ability Power, which is seriously no joke. If the enemy doesn't focus you, you're gonna wind up with so many kills you'll be able to pin down targets within a heartbeat.

Viktor

spa

I like to get this relatively late, as opposed to Hextech Ray-Viktors who desperately waste 1000 gold on 45 Ability Power and the added not-that-strong burn effect on Hextech Ray. You won't need to upgrade The Hex Core as soon if you're going to get an Augment: Gravity out of it, so after the Deathcap will do.

Viktor

spa

This is a really powerful item, providing a nice amount of extra HitPoints which help keep you alive just that extra bit longer, but what it's really good for is that you get a 1000+ range slow on it. You catch the enemy carry off-guard with it, they're either dead or out of Flash, simple as that.

Viktor

spa

I get this relatively late, since I don't have mana issues with the Crest of the Ancient Golem, and have maxed CDR with it too. It's just not worth investing all the money into. On the other hand, once you get this and an Elixir of Brilliance, you'll have capped CDR as well, paired with the perfect amount of mana sustain you'll need. No need for blue buff no maw.

Viktor

spa

Last time I got this far in game, I got me an Archangel's Staff, for teh trullz. A WotA would have been way better in the last two team fights, but what the hell :P A WotA gives plenty of sustain - which you really don't need anyways, it won't save you any time soon - but also a nice aura that stacks with a second WotA-keeper.

Viktor
Viktor
This girl is a major pain in the ***. The fact that she's got a guaranteed stun, and rediculous figures on all her spells makes her a very difficult laning enemy. If you have a jungler - in the current meta, this is pretty much standard - ask for a quick gank to shut her down. A well-coordinated gank can set you one or two levels ahead, meaning you have a nice poking advantage. If she has her stun up, only take the last hits you're certain to get. Don't overextend against her. She will punish you.Viktor
Viktor
This guy... he's one of those pesky guys that won't leave lane. He has good sustained damage with high figures, and a consistantly big shield that makes damaging him a hard mission to accomplish. Just take a few things into account. His Siphon of Destruction is very easy to predict, and Mace of Spades only hits the first few targets. He is mostly cooldown based, so wait for his sh!t to go on cooldown and take his health.Viktor
Viktor
Sweet Jezus, this woman is hard to pester out of lane. There's no end to her sustain. It's not funny, for serious. She farms easier than any other champ I've met in a long time - even though Morg is as old as Planet Earth - and her ultimate is just way too powerful. If it seems like I'm looking for excuses, then I'm sorry, but that's just what Morg does best. Poke with Puddle 'o Death, Snare, Ult, Killing Spree. You can't poke her down. Damn it all.Viktor
Viktor
This is... a worthy lane adversary. She has good poke - Sigil of Silence vs Power Transfer), but she has a good burst combo. Aside from that, she's useless. If she wastes her ultimate, her damage output drastically cacks in, meaning that with a good burst combo, you'll easily take her down. Ask for an early Blue, and primarily use Hextech Ray to poke her, since using Power Transfer puts you in range for her combo. She is a worthy opponent though. Don't take her lightly.Viktor
Viktor
Another worthy lane opponent. He has a good CC with his Null Sphere, but that's all. He is one of those melee range mages, which are relatively easy to counter due to your more-than-reasonable ranged poke. His Sphere is about the same range. If you're faster, the shield will block some of the damage. He's got better burst, and can just recuperate the mana via basic attacking, which is a setback to you. Your poke is better though, so it's an iffy-kinda lane.Viktor
Viktor
He is... not strong, but he's got good damage. His Lay Waste deals good damage, and if you walk into it, you'll get the hurt. His Wall of Pain really hurts your movement and all you can do, not to mention his damage against you increases vastly. And if he pokes you down, there's his ultimate. Luckily, you have more than recent poke too. Just a shame he has mana recuperation in his kit. +1 for Karthus. You should be able to win it though, you have better sustained damage, I think...Viktor
Viktor
Another worthy lane opponent. Don't underestimate his damage. Even though he has no guaranteed single target nukes, he has large damage in an even larger AoE, meaning that even though he has reasonably slow projectile speed on his Barrel Roll, he can still hit you for a large sum of damage. He has better ratios all-round, meaning that if he gets a full combo off, you're in pain. You can outdamage him, since his mana costs are really high, but don't get overextending. That sh!t's dangerous.Viktor
Viktor
Ahri is a fairly easy lane enemy. She has no direct single-target damage, meaning that with a pair of Boots, you're mostly safe from her damage. Your Power Transfer should take care of anything she might throw your way, but her damage is never guaranteed. She has no stun, you do, she has no single target nuke, you do. You should have the upper hand. If an enemy jungle gank sets you back a level or two, just keep poking. She will never outdamage you, only outmobilize you.Viktor
Viktor
Haha, this guy is so easy to take on. Fizz should be no challenge what-so-ever. He'll mostly use his Playful / Trickster to dodge your Power Transfer, and if he doesn't, he gets smacked in the face with a lot of damage. His only poke is from melee range, meaning that a well placed Gravity Field mitigates his damage to a crisp. Your full combo destroys him. Don't get cocky though. Playful / Trickster is not his only gap closer, and Chum the Waters provides good entry CC.Viktor
Viktor
If you lose this lane, you should sell Viktor back to the League. Okay, maybe not that. But she only has one poke that is guaranteed to hit you, and if she uses it, Flash is her only escaping tool. You have really strong poke, she has meh poke. You have a full-percentage SpellVamp spell, she only has one when she sacrifices her only escaping mech. And her ulti is easily negated bu yours - remember the short-duration silence? This lane is a free win.Viktor
Viktor
I don't find Vladimir a threat in mid lane. He has reasonable damage, but... it's kinda hard to explain. You easily outdamage him, if you poke at him consistantly. He can regain his health, you see. That's a pain. His main damaging skill costs no mana, and doesn't give you time to shield to counter it. He lacks a continuous stream of damage though, and aside from Flash, his escape mech costs him a sizabe amount of health. You should win this, but select Vladimirs know how to counter you too.

Viktor

In the early game, I 'want' you to go as offensive as you can. Power Transfer has a really low mana cost throughout the entire game, and combined with more than reasonable damage figures, this spell will make you win games so much. After Lv.5, you'll be very hard to take down. I can imagine enemies with channeled abilities who will get destroyed by Arcane Storm (the silence, anyone?), which means that after Lv.6, you'll pester anyone out of lane, if you manage your skills properly.
ViktorViktor
In the middle game, you can really assess your lane if you - once again - manage your skills properly. Fights and engages can really turn around thanks to a good Gravity Field and Power Transfer. Your ultimate should only go when you know your enemy has wasted all their escape mechanisms (Kassadin has his slip-'n'-slide, LeB has her poof), and then herp them. Also, roaming becomes very powerful if you know when and where there are enemy wards, so coordinate ganks with your team.
ViktorViktor
Now you're a beast. Your single target damage is very powerful, capable of destroying single targets is managed properly, along with good sustained team AoE damage (your ultimate, and an AoE stun). You should really focus down the squishier carries (Vayne, Tristana, the lot) before switching your attention to any others. High priority targets often gove you the most gold, and are often - duh - an important part of the enemies' damage.

To conclude things: Viktor is a very powerful mid champion. He has good sustained damage, reasonable poke, and more than reasonable Crowd Control, even in later stages of the game. His disruptive and self-sustaining power is so powerful, you shouldn't have such a hard time taking anything on (unless you're getting focused. At 6300, I find him a good buy. I purchased him via le bundle, but I'd definately pick him up if I hadn't and I knew I'd be this more-than-reasonable.

Thanks to Ciderhelm for the good info on his abilities, and JhoiJhoi for the allround info to making a guide, and the divider ofcourse ^^

This was Emo. Signing off.

03-04-2012
~ Published guide :)

10-04-2012
~ Added the 'Laning Setups' section, by popular demand :) Should make things a bit more clear. Note that it is purely from my PoV, so it's open for debate.

12-04-2012
~ Added Morgana to Lane Setups. F!ck.