Hello everyone, welcome to my first build guide! This one I am making for Viktor, The Machine Herald. Viktor, I find, is fairly squishy early game without runes, but becomes very tanky late into the game.
I do hope you enjoy, and please, don't dislike before trying it :)

For the runes, I take a few defensive Seals with an option of damage Quints and defensive quints, and damage Marks and Glyphs. The defensive Seals you can choose between magic resist and armor.
The reason I chose these runes is because it gives Viktor a damage boost, and also allows him to be less squishy early game, too.

My masteries are fairly straight forward, it's pretty much pure magic damage with a bit of attack damage mixed in. However, if you find yourself squishy, you can swap Alacrity and Weapon Expertise for a few points in Durability and Vigor.

For items, I take either a ruby crystal if the are ad / ap for early survivability, and I take a sapphire crystal against tanks so that I can constantly harass them, stopping them from getting a good farm.
Next, it is a catalyst the Protector, building of into a Rod of Ages if you are solo mid or top, and Banshees Vale if you are duo top or bot, as I find that with a team member, you can output enough damage onto one unit that survivability is better than a bit of extra damage.
After that, it's some of those lovely Boots of Speed, and the rest depends on the enemy team comp.....
AD:
Here I will get Augment: Power as the shield plus speed boost can be enough to escape unharmed. Also, I would grab a Lich Bane after this so that I can land a power full attack before running from a AD. Rylai's Crystal Scepter is for the extra hp and ap to survive more attacks and deal more damage, Zhonya's Hourglass for the godly armor and so-so ap. Ionian Botts. Why, though? Surely you will want sorcerers shoes? No. The cdr is amazing against melee, as it means you can poke them with your abilities just as often as Twitch can poke you with his auto-attack.
AP:
Augment: Death, this is so you can outrange what the AP champs on the enemy team can throw at you with Death Ray, and deal FORTY PERCENT MORE DAMAGE. 40, that is loads. So, at the end of the AP build, that would equal, 252 WHOLE damage over a few seconds, that's like, another ignite, sorta. Next is Abyssal Scepter, for MOAR ap and magic resist against the ap champs, then merc treads, fairly obvious.
No Anti-tanks on your team / loads of tanks on their team:
Augment: Death, I have explained this, void staff, AP and HUGE magic pen. The moment a tank starts doing badly vs Viktor, he will get magic resist, so with void staff, the more magic pen he gets, the more damage you do. Will of The Ancients / Abyssal Scepter. If the tank is someone like Cho'Gath who does loads of magic damage even with a full tank build, then you will want Abyssal Scepter, otherwise get a Will of then Ancients for the sexy Spell Vamp aura. And then Sorcerers shoes, for more AP.

After all this, you will want the almight Rabadon's Deathcap. You can also build this amazing item right after the boots of speed, so all your ap items are 30% stronger.

My skill sequence is fairly basic, Death Ray deals 10 less damage than a Power Transfer, but it can out range (I think) all ranged basic attacks if you point it away from Viktor, so it is good for early aggression. If they are playing a melee however, then you can take power core, as when around rank 4 or 5, it returns a lot of damage as shield, so if the melee thinks you are trying to poke, you can just run away and any attacks they try to hit you with will (hopefully) be absorbed by the shield.
Now, gravity field is a crowd control, so you just want it for the cc, not the amount it cc's them for, so you get it just before your ult (and maybe the junglers ult) for good ganks and / or kills.
Lastly, is your ult, which should be gotten at the obvious levels of 6, 11, and 16. Your ult moves faster depending on how close you are too it, so unless the enemy already has level 2 boots, sending it past the tower whilst you farm minions probably won't kill them.

Summoner spells are mostly down to personal preference. I take flash and ghost for getting away, as ganks can be common when you are dominating your lane, and you might be against Twitch or Ryze (Who are the only champions in the game I consider to be 'OP') who can kill you easily and thus you may need to get away.

After you get Needlessly Large Rod for Rabadon's Deathcap, you should be able to one shot minions with your E, thus you can use it really good for minion farming, if you can aim. When destroying towers you can place a gravity field just before the enemy minion wave arrives so they get slowed and maybe stunned, stopping your minions from getting distracted and allowing you to do more damage to your tower. Your Q can be good for last hitting Siege Minions too.

That's the end of this guide! I hope everyone likes it, and I am sorry if it is not the best, it is my first guide and I don't know any of the coding that adds links, other than that, enjoy playing!