I have a pretty good feeling if a strong player gets Marth, they have a better chance of winning. He's mobile, strong, has good range, and is good in FFA's. Along with him being mostly the same in previous Smash games it seems like, so I believe the player will have an advantage.
Sadly none of the pro players seemed like they were picked from Brawl, so I think some characters will be less likely to win. Namely Zelda, Olimar, Sonic and Maybe ZSS.
Link and Mac have terrible recoveries, so those two will have a hard time making it past FFA's.
DK is typically a hard character to master, but I think he would be great in the FFA stage with his new super armor moves.
Bowser will be played by DJ nintendo, most likely, so that increases Bowser's chance of winning.
Pikachu has always been an easy character to spam with, and since they'll all be playing a new game, it could take players longer to learn how to deal with it.
From what we've seen, I don't think Villager or MegaMan seem like great choices for FFA, but I could be wrong.
The character I think with the best chance of winning is Marth, but I'm rooting for Pit or Villager