Why pick Viktor on top ? Because it's super fun and strong ! This champion can influence the game by many diffent ways : Ganks, split push, temfight, turtle, objectives... Well played he can carry alone a whole game. Let's see more !

Viktor has a major problem : He needs a lot of gold to be efficient. Here comes the first rune : Kleptomancy. First you will earn gold each time you use your Q on your opponent. You have to abuse this to get strong as fast as you can ! Also, this champion is very squishy, especially compared to other classic toplaners (hello Sion, Urgot, ...) Klepto will provides you little help about that : extra potions, boosts... I insist on that point : be sure to proc Kleptomacy as much as you can. Also, be sure to don't go too far or too close of your opponent : He may be able to punish you very hard, and early ganks on you are extremely dangerous.
Let's talk about the other runes. First, magical footwear : This will give you free boots at minute 10, with an extra bonus of 10 move speed. As i said, Viktor need a lot of gold to be efficient; by giving you free boots, you will save 300 gold, what is really interesting especially in early game. Also the bonus on movement speed is really cool because Viktor is one of those champs who loves MS : You can dodge easier (what is your only way to escape since you have no dash / tp), cast you spells faster,...
The biscuit delivery is important because it offers you extra sustain during laning phase. Combined to the klepto, this really helps you holding your lane. Also the mana regen is extremely usefull in early game.
Let's talk about the cosmic insight and why i like this one. First, be sure to know that Viktor is absolutely nothing without his spells : the extra 5% free CDR and max CDR litteraly can make the difference between life and death. The 5% CDR on your summs is pretty cool too, but the bonus CDR on active items is pretty useless, except for your trinket.
Both of the secondary runes are pretty much classics : manaflow provides you mana and mana regen and transcendance will give you the 10% CDR you need to reach the max cap.