One is your wife/husband/spouse.
One is your best friend.
One is your co-worker.
One is your pet.
For a pet, please pick a non-humanoid champion (Champions with highly bestial traits like Ahri are okay.)
Validating your ideas makes it more fun!
Picking a specific skin for a champion is great (I.E. Corporate Mundo for a coworker)
I would marry Cassiopeia. That snake tail would be great for spooning. I mean, what?
Blitzcrank would be my best friend. Many laughs would be had rocket-grabbing cars off the street. Or maybe I'm just a sadist.
Caitlyn would be my co-worker. Even head, highly intelligent, awesome top hat.
And finally, my pet would be Kog'Maw. Do I really need to say why?