My list is a little outdated, haven't been playing a lot of teams lately, and I'm probably forgetting to mention some people, so don't take this as exhaustive.
ANd it's in no particular order.
First, the things I like in a teammate are really quite simple. Cooperation is the key. I also like to know that I don't have to hold my teammate's hands and he can hold his own, that way we can both focus on winning. That's not something that ever really comes up, though. The other thing I like in a teammate is when we don't have to talk about tactics, we can just play the game.
So here we go, again, in no particular order.
Your_mom: very cooperative, and we've played together enough to know what each other does in a given situation, and we're pretty good at exploiting it.
Chills: first time I played with him was when he was on my team. It took a few matches to get the hang of it, then we just started winning.

nemo: (this is what prompted this post, actually) excellent teammate. Of course, as an opponent, he's really tough precisely because he knows exactly what you're going to do at any particular moment.
Yaza: favorite thing to do with yaza on my team: seal him in when he attacks somebody. 9 times out of 10 he'll kill his victim and come out through the hole, and those same 9 times out of 10 his victim won't be able to escape.
I recall a match with both your_mom and Yaza on my team, well several matches. I don't recall ever losing, nor do I recall ever having a close spread.
Ok, now what's your favorite position to play when you're on a team? I like to be the point guy on a 3-man team. Sure I die about half the time at the start of the round, but the other half of the time I take out like 3-4 players right away and come back for more. It's the best when both of your other teammates hurl you into the middle.