I would probably only like a team mate to be rosalina or peach. Only because I know all of their moves and how they would be playing. Stuff like that. I love olimar but I wouldn't trust a team mate using him. CPU or real.
I'm not really too fussed, it's just depending if the team member is going to contribute properly, I love having Jigglies that are good though, I'll grab an opponent and I keep pummeling them till the Jiggly rests on them \o/
As long as my teammate is good I don't mind which character I team up with. However, it is very satisying to grab an opponent and have my teammate use a falcon/warlock punch. It is still satisying even if team attack is on and it KOs me as well.
I usually use Ike, so I like to be paired with teammates who provide some speed and projectile support. Fox, Falco, Pit, Link, and Toon Link more or less fill those roles. I don't like being paired with other heavy-weights (Bowser, Ganondorf, Dedede) because fast characters on the opposing team can just run around and bombard me and my teammate with projectiles and it is easier for the other team to create 2 on 1 situations.