Galleom

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: Ike, Marth, or Meta Knight for the 1st fight(you choose two[three if no 2P]). Lucas or Pokemon Trainer for 2nd fight(you choose[you'll play as both if no 2P]).
2P's Character: Ike, Marth, or Meta Knight for the 1st fight(can be any one of the other characters 1P didn't choose). Lucas or Pokemon Trainer for 2nd fight(you choose one).

Taboo (Tabuu)

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: All (you choose five[six if no 2P])(You must of picked up their trophies in order for you to choose them)(Wolf, Toon Link, & Jigglypuff can only be chosen for this fight when you beat the game once and defeated them in the hidden doors on The Forest area, The Ruins area, and the Swamp area)
2P's Character: All (can be any of the other characters 1P didn't choose)(You must of picked up their trophies in order for you to choose them)(Wolf, Toon Link, & Jigglypuff can only be chosen for this fight when you beat the game once and defeated them in the hidden doors on The Forest area, The Ruins area, and the Swamp area)

Master Hand

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: Any (you choose before starting Classic Mode or Boss Mode)
2P's Character: Any (you choose before starting Boss Mode)

Pokey Minch

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: Ness or Lucas (you choose[you'll play as both if no 2P])
2P's Character: Lucas or Ness (the opposite of the character you've chosen)

Meta Ridley

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: Donkey Kong, Diddy Kong, Captain Falcon, Pikachu, Samus, Pikmin & Olimar, or R.O.B (you choose three[four if no 2P])
2P's Character: Donkey Kong, Diddy Kong, Captain Falcon, Pikachu, Samus, Pikmin & Olimar, or R.O.B (can be any of the other characters 1P didn't choose)

Ridley

Note: I know the this guy doesn't appear in any sonic game, but I though you should know the bosses from Super Smash Bros. Brawl since Sonic is appearing as a playable character. I don't have very much info on this boss, so I'll put in what I can and put more in later. You can battle this boss as anyone in Boss Mode of the game (You must unlock it first though)
1P's Character: Pikachu or Samus (you choose[you'll play as both if no 2P])
2P's Character: Samus or Pikachu (the opposite of the character you've chosen)
Video of Battle: Click Here to see. (Note: I found this video, but I did not make it.)