Hi, I'm Ksarrow and this is my guide.
Take a few minutes to read my guide. It won't be that long.

Why I took these runes? Magic Penetration, Mana Regeneration, Scaling AP and AP?

Magic Penetration..well..you need it. If the enemy build magic resistance and you don't have any Magic Pen, then you are lost. I only use flat Magic Pen. in my build, because my masteries and runes are good enough.

Mana Regeneration: Viktor is a mage -> Viktor uses Skills for damage -> His skills cost mana.
And trust me, his Mana Pool isn't that high in the beginning, like his Mana Reg. But late game, you don't have this problem, because his Mana and Mana Reg. are growing per level.

Early game.

Beginning: Boots + 3 Health Potions
For more movement speed, and sustain.

First Recall: Augment: Death + Crystalline Flask
For early damage output and even more sustain. Because you have no life.

Second Recall: 2x Doran's Ring
For the life, little AP boost and the mana reg.

Mid Game.

Rylai's Crystalscepter.
Life + AP, and your skills can slow. You can chasem the enemies easier with your Q and E, and it is harder to escape from your Ulti.

Rabadon's Deathcap.
AP. Core Item for every burst mage with AP scaling.

Zhonya's Hourglass.
AP + Armor. You know that you do damage. If the enemy knows, don't let them focus you! In teamfights, activate it only when you are in this situation:

1. You bursted your whole combo and now you must wait for your cooldowns.

2. You unlucky bastard stand in the middle of the whole teamfight.

After you activate it, they can't focus you for a few seconds. Sometimes, the seconds decide between alive or not.

Liandry's Tornment.
AP + Life + Flat Magic Pen.

Life is important. AP is important. Magic Pen. is a nice bonus, for bursting down their carries. But I love the passive. It's like a Madred's just for mages and as DoT. And don't forget: it does the double amount of this damage, when the enemy is slowed. It works pretty well with Rylai's.

R>E>Q>W

Power Transfer = One target. Fast projectile to the target. Flies then back. Gives you shield. Can save your *** e.g. against Ignite.

Gravity Field = AOE-slow/stun. Great for capture a bundle of enemies at once. Nice anti-ganking skill against melee champions. CAUTION: Stuns only when you can slow. So Master Yi with activated ulti won't be stunned.

Death Ray = Vector-AOE. High damage and Viktor's signature skill. Easy poking and farming from far away. And if you bought the right Augment, they will BUUUUURN! Btw, no casting time. So you can run away and kill champions at once.

Chaos Storm = AOE-Silence-Burst-DoT-Cloud. Don't underrestimate the DoT. It does more damage then the initial damage.

Pros:
-high burst potencial
-very underrestimated
-easy farm, thanks E
-gains AP per level without runes
-pretty much AOE
-hard gankable
-can shield himself
-CC: slow, stun and silence.

Cons:
-mana hungry
-slow
-need fast combo
-squishy at the beginning

Don't underrestimate Viktor.

Have fun.