I'm surprised at the number of people claiming ZSS would be good because of her spacing game.
Yes, ZSS has an amazing spacing game. Her plasma whip however, doesn't really work in 2v1. At all. It comes out slow enough that a quick opponent can see the start up animation and get inside her range before it fully extends which leads to the bigger problem with the whip is that it has a blind spot, and a rather large one at that. What this means is that where an attack like this would normally knock someone who rushed in away and keep you safe - it doesn't do this unless you hit them with the end of the attack, anyone who gets into the safe spot is pretty much free to punish you. This is problematic in a 2v1 because very rarely will a pair of intelligent opponents approach at the same pace from the same direction. If their forced to approach you from the same side, one is likely to be approaching on the ground and the other in the air. This essentially means that one will end up in the blindspot and punish you.
Punishment in a 2v1 is an especially bad thing, especially whene your on the light end of the scale like ZSS.
She does have moves that compliment a 2v1 setting but I'd hardly call her good at 2v1. Down smash is phenomenal, if you land it, it has a large enough hit box to catch both players, and even if you don't hit both you can leave one in a stun for a few seconds while dealing with the other on more even footing. Not ideal, but then again, this is 2v1 we're talking about.
If I were to bet on a best competitor for 2v1: Either MK or Snake. MK would have to play a more evasive game, but if the MK was cautious he wouldn't have too much trouble gimping (returning to the stage would be just like recoving vs. one opponent), his tornado is one of few moves that could actually mess up two people at once. Down smash is quick and hits both sides for a quick "get away from me" type move. Yeah, MK is fairly light and punishment sucks in 2v1, but as seems to be the brawl modo(sp?) "don't get grabbed".
Snake on the other hand I can see working only because the man lives forever. He's a forgiving character in a high punishment scenario.
Mind I play neither MK or Snake, these are just the characters I'd throw out if I had to guess.
