of the 12 levels I've played and beaten so far, I'd say that Mystic Mansion was the coolest. In terms of bosses, I'd say....hm....probably the Team battles because it's fun to beat the crap out of the more annoying teams (Rose and Chaotix-Rose because they're so crappy, Chaotix because of the aggravating levels where you constantly need to stop and backtrack). Then again, the Robot Storm and Egg albatross where still fun....hm...... and as a side comment: All special stages after Bingo highway (except Bullet Station, that special stage was really easy) are evil. It took me 20 minutes of non-stop playing to get Bingo Highway's Emerald, and I cannot do that again.
