1) Darius. Slam dunk go!
2) Kassadin. Really love his mobility and his general playstyle. Nothing else really like it in this game or other games.
3) Jarvan IV. Really cool champion design. Love the knock-up spear/flag combo and his ultimate.
4) Varus. VERY cool theme and design. Kind of the archer/dark knight hybrid, never seen anything like it before in any form of media.
5) Vladmir. Cool design, crazy sustain, interesting playstyle. Love this champ.
There are so many more I really like too. Graves, Blitzcrank, Malzahar, Alistar, Yorick, Riven, Irelia...