Piranha Plant: Did not know what to do at first.

Thought you had to grab it and quickly toss it down while he was spiting them back out. Boss was so-so.
Hammer Bro: Easy until last pattern. Totally was first boss material.
Dry Bones: You having to run around and use magic to destroy the boss was more different and creative and was cool. (Love how he falls apart at the end.)
Kamek: ? at obstacle course... Easily destroyed him when I get to him... Major dificulty spikes... equals least favorite boss.
Bowser: Destroying the first two forms is easy but the final form...dang. Why would Bowser want to turn himself into a tile anyway. All Mario needs to do is run up, grab him, and toss him on the floor. It would be: *smash* one heart gone *smash* two hearts gone *smash* boss defeated. Problem solved.
Ranking:
1. Dry Bones
2. Hammer Bro
3. Bowser
4. Piranha Plant
5. Kamek