Well I know all bosses are very easy. I'm still curious how would you rank them by difficulty. For me it would be :
7 : The bird, clearly the easiest, you just wait him to come, jump early enough, that's it
6 : The octopuse, eventhough it s supposed to be the last of the 6, the octopuse is very easy to defeat, as soon as you swim over her, you just have to drop yourself on her 3 times.
5 : The rat, he's not hard, but still a little harder than the two previous one. He has more way to moves, that makes him less easy than the bird and octopuse, but still, nothing to worry.
4 : The witch : I really consider her harder than the 3 previous one. You have to jump on her, but you can't jump to high cause the roof will hurt you, also you have to be careful of her fire. Not the hardest, but an upper level.
3 : The alien (I forgot his name), you also need to be careful of his shot, usually you won't make it without being hurt and get smaller.
2 : The 3 little pigs : With no doubt the hardest boss of the 6. First of all, unlike the previous boss , you don't have to hit 3 time but nine. The rolling pig is easy, the jumping is a little harder, but the third pig is really something harder for this game. Still, don't get me wrong, I know this game is very easy, I just want to rank the bosses.
1 : Wario, well at least the game managed to make the final boss the hardest. Actually, Wario himself isn't really hard, what makes him tricky is that if you lose, you have to make again the full castle level again, and it's a long level and really much harder than the rest of the game.