It's very simple. Imagine (or better watch some clips) of your most hated champions killing each other and see who you're rooting for. If you never root for one particular champion - congratulations this is the one you should ask Riot to delete from the game.
In my case, however, I couldn't decide what I like more: Blind Dumbass being outplayed by a 10HP Japanese-Flute-Player-With-Free-Shields-Because-Why-Not or a 0/10 Powerspike King being kicked into the fountain by the aforementioned sight deprived individual. Thus, they are both my number 1 as my perfect scenario is when they both die in an equally humiliating way.
Please, share a similar thought process and have fun!