Xerath Viktor or Kassadin

I think Xerath is easier than VIktor so generally I'd say Xerath, but if you're willing to devote time and practice into him Viktor is a better choice.

Kassadin is in a wonky spot right now. He needs a bit if lovin. What do you already play?

Kassadin is extremely fun to play, but he is extremely feast or famine - If you are able to survive and maybe even kill in lane - you will dominate. If you fail even a little - you lost. (For that reason I usually take him top, where he has a bit of an easier time)

Viktor is fun, but he's... Well... Not very strong. He's not VERY weak, since his buffs, but Xerath is overall a better pick - as in stronger. If you're interested in winning - Xerath. If you like to have fun, but risk losing - Kassadin. If you are a masochist - Viktor. And if you enjoy him, pick up Morde next.

Xerath is the easiest and most safe to play of the 3.