Doubles - no question. It's the perfect amount of space for there to be someone setting up and someone shooting goals, without getting clogged like in 4's.
4's can be interesting but its definitely chaotic and extremely hard to get anywhere in. I do enjoy playing doubles (as you have introduced me to it). However, I'd have to say 3's are my favorite since I feel it is the perfect balance.
I like 3v3 then 2v2 in competitive. Also will play these modes in non ranked when no friends are playing my only problem with non rank is I play high ranks in comp. And the non ranks seem so bad I sit back and watch as everyone is trying to hit the ball and laugh !
2v2, just cause i feel like i can get the ball the most and team play comes in quite well
I still do 3v3s though just cause there's lots of action there