In this Build, I will explain the reason why you should play Viktor and how can play it.
I main Viktor since I started LoL. I always found fun to play it.
I know my build page will be horrible to see, but i don't know how can put image like for the spell and so on. :/

I°) Runes
II°) Masteries
III°) Items
IV°) Spells
V°) Pros/Cons

Against AP MID LANER: I take Seals of Magic Resist and Glyph of Magic Resist. With AP Quints and Magic Penetration for the Marks.

Against AD MID LANER: I take Seals of armor and Glyph of Armor. With AP Quints and Magic Penetration for the Marks.

Other kind of runes: CDR Quints,Marks of Magic Penetration, Seals of Armo and Glyp of Magic Resist.

Offense Tree

Tier 1 - We get 4 points in Sorcery because it's the only AP choice there is. 1 point in Butcher to last hit easier.

Tier 2 - Same as the 1st tier, we get 3 points in Mental Force for the same reason. Feast gives a decent sustain with 36 hp regen / minute (If we don't fail any CS).

Tier 3 - Arcane Mastery is the AP choice(6AP). Executioner is important in order to kill low-healthed enemies.

Tier 4 - We only get 2 points in Arcane Mastery because more 1,5% AP isn't important compared to with Dangerous Game .

Tier 5 - Devastating Strikes is very useful for the magic pen, and viktor auto-attacks, so it's good to have that armor pen. Arcane Blade is useful with Lich Bane and Siphon Power.

Tier 6 - Havoc gives extra physical/magical damage. It isn't much, but is still useful.



Utility Tree

Tier 1 - Phasewalker is very useful because one less second is very important when you're backing, specially if your base is getting backdoored. Meditation is something we need due to Viktor's ability to run out of mana easily in the early-game.

Tier 2 - Summoner's Insight is very good because you'll be flashing alot. Strength of Spirit is quite decent since you'll end up with more than 1700 mana. Alchemist is very good since you'll always want to have potions to be able to outlane your opponent.

Tier 3 - Since the buff duration was nerfed, Runic Affinity isn't as good. However, Culinary Master gives you instantly 20HP and 10Mana, which is very good in the middle of fights.

At starting, most of time I take Doran Ring with two potions. Then, if you can back when you have 1400po, go back to buy a second Doran ring plus the upgrade on your E.
After you will need boot. I prefer to buy Lucidity boot to get CDR.
The first item I purchase now is Luden's Echo. This item is very good to push and harass your enemy and between you can walk quickly !
After it, I like to build Lich Bane to make more dammage on your Q. Then you will need the second upgrade on your Q to can chase and kill your opponent.
After build these items, let reach Rabadon's Deathcap and void staff. But you can take Void Staff before Rabadon if your enemies has a lot of Magic Resist. And to finish, buy your last upgrade.
About Elexir, I prefer to take the Elexir of Iron as I don't use any defensive item.

Well you can think it's not good to don't get at least when defensive item, but i think if you do good placement in teamfight, you don't need it as Viktor is very strong in late game.

You need to skill your E, then Q and your W.
But at your level one, take your Q first. You can attack enemi without lose too much HP thanks to your Q's shield.
Your E will be very usefull to harass and push. And in teamfight, to can do AOE damage.
Your Q will make awesome damage with Lich Bane. And with the Q's upgrade, you can walk quickly to escape or chase.
Your W is usefull to trap enemy or run away when you're in danger. Always use it in teamfight, it can give to your team a serious advantage.
Your R is mostly to finish your enemy, but never forget it's a silence, so you can stop Warwick's ult or other like Nunu. And always think to move your ult, you can think it doesn't do any damage after put it, but it did enough to can kill one with no much HP.

Pros:
+ Nice damage
+ strong in mid and late game
+ Nice damage AOE
+ Nice CC with a silence
+ Can run quickly with this build

Cons:
- Doesn't have nice escape
- Shorter range than other AP carries
- Can be manna hungry
- Not easy to master
_Not as good against melee champions early-on.