By the way, if anyone has any nominations for a BIS, you can start voting for those too.
Best first level. I'd vote for SMB.
Best credits. I'd vote Mario DDR or Wario Ware Wii because they are interactive.
Best remake. For example: SMB -> SMB Deluxe, SMB Allstars, etc.
Best flying game/level/world. This would be good since you already did a water level, but there are less games with flying.
You could also do followups to previous BIS. For example, since SMB3 won best powerup, you could do a best powerup from SMB3.