There are 4 main runes for viktor spellbook(my fav),aery,electrocut,phase rush
-spell book is great for every machups because you can take any summoner spell you want anytime but its not so useful in early
-aery is great for lane bulling if you dont want to figght with enemy just poke him this will be very useful and will deal lots of dmg
-electrocut is great for fragile enemies when you just want to kill them as fast as posible before they deal you lots of dmg its very powerful in early but its not so good in teamfights
-phase rush is great if you want to have better positioning its kinda great all the game

Very important thing is that you have to emember to not burn your summonerspells in late game to just get kills if this kill will give you a tower great burn your summoner spell if this will be needed but dont flash to enemy to just kill him its just not worth in late game now all the summoner spell that you can use on viktor

-ignite its great to scale in the early if enemy dont have huge hp and if enemy have high healing like vladimir
-barier its great vs assasins with high dmg like talon because you can shied and they maybe wont oneshot you
-cleanse its great if enemies have many stuns in team so you can cleanse in a teamfight and just kill all enemies and save your team
-ghost its great if you want have better positioning so its kinda great for every matchup
-teleport you kind dont want to take tp when you are on mid but if you play with spell book you can use it to roam or help your team in temafight
-exaust you kinda again want to take it only from spellbook but its realy useful for very mobile champ because if you use it on for an example akali she wont be useful in fight and you can easly kill her
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thx for reading my guide hope you will enjoy plaing viktor