The bat. lol. Go to the very far end of the map and let him walk to you. most of the time he won't fly away. If he does, run to the other side and do the "A,A" spin attack when the small bats come.
Second best.. I'd say it's the corn boss, since you can get up to 5000 exp. However, it always goes back into the ground after like 4-5 seconds. So, it takes like 8-9 minutes to beat.
I believe any other boss pretty much sucks, and catfish as well as barbarian boss can crash your game, or at least

it up. I tried it on the Barbarian Boss, and my screen got all screwed up until I reloaded game again.