Which videogame bosses would you say had the most wasted potential?

Sonic Colours's final boss is seconded. It has two bitchin' pieces of music, and the last one in particular you will hear less than half of during the fight. It also only seems to use three of it's attacks. It's a shame really as the stage is set and the atmosphere is epic as hell.

Also Metroid Other M's final boss against MB - it shouldn't have even been a boss. I understand why it wasn't a full fight as we already had the Metroid Queen, and MB always, to me, seemed purely exposition. It's just it was so pointless having to do that aim the shot thing. It was unneeded.

You don't even get the chance to shoot MB yourself. Yes, you yourself, the main character, doesn't even conquer the villain in the end. Instead, a character that was just only introduced 5 minutes ago shoots her!