I only play internet flash games and the occasional freeware so my references are obscure but definitely the baron from sonny and sonny 2, especially sonny 2. he's just so powerful you feel you're never going to win and keeping yourself alive is a struggle, let alone hurting him... also from the freeware game cave story, which all of you should play (be sure to find an english patch), pretty much all of the bosses are memorable but the most striking would be misery, ballos, monster X, and maybe balfrog. shoot the other games with memorable enemies are slipping my mind... hmmm