Viktor is a very versitile mage, he can be built in nearly any style whether it be tanky or high damage output or any other styles, it is all the players choice. this is my perfered build which is a high damage output build.

PROS CONS
High Damage when spells on CD he is useless
Good auto attack range only really get to buy 5 items
Stun can help you survive against gankes

For the Runes i like to go with a decent amount of ap do help do high damage at the start of the game, i also like to use the the magic penetration runes just incase the enemy laner is got magic resistance runes on. Runes can determine whether you win or lose your lane, when i first started playing i just wanted to buy champions and skip out on runes but i have learn by experiance(losing lanes) that runes can give a huge advantage.

For Viktors masteries i put 21 points into offence section i focus the AP assisting masteries as much as i can but i also use Cooldown reduction masterys in the offensive section. i also put 9 points into the Utility section. i put the points into the max mana and mana regen sections then recall CD reduction and buff time increse.

I personaly like to start the game with the dorans ring for the damage and Hp addition. i then move onto boots of speed. i then go straight for the rod of ages for the stability in lanes. then finish off my boots with the sorcerer's shoes,Then buy the rabadons deathcap for the huge ability power gain, Then i like to buy the power augment although its not in this build. then i buy the morellos evil tome or the void staff. to finish the game i buy a zonyas hourglass for the armor and the extra ap.

this is my first build so if u can give and helpful feedback on how i can improve it is apreciated, i know it was really short and i will try to make them longer when i make more builds. Thanks