The fireball is very powerful, as its special attack is very cheap and means you don't have to worry about running out of bombs. But I usually do best with the vampire bat, which is really powerful if you're careful and make sure to buy a health upgrade or two as soon as you can. The slime is strong as well, killing most things in one hit is very useful.
I think those are the strongest three, but it depends on how you play, what upgrade you like, and a little luck.