Hmm..this is a hard one. I think my favorites are Gohma and Kalle Demos from WW, and Ghirahim from SS. Gohma and Kalle Demos are fun and easy, and Ghirahim is just hilarious in my opinion. But extremely epic.
Favorite Boss: Zant (I know everybody is going to look at me like this :S, but I actually enjoyed his battle!)
Least Favorite Boss: Gohma *OoT* (I didn't really like him, it was to bland. Don't, like, online murder me because of my opinion)
Most Challenging: Ganondorf *Tp, horseback* (I kept falling off, teehee )
Least Challenging: Diababa (I gave him one whack and he died, I mean, what the?!)
Coolest: Puppet Ganon *TP* (How many times have you fought Zelda herself? Hmm??)
Lamest: I have to think about it...
Ohh man this is hard! I like alot of them!
My favorite at the moment is probably in SS. Ghirahim's last battle, Demise boss battle & that gold thing with a dozen sword you take with the whip.
At the moment, my least favorite is the first minish cap boss battle. I dont remember the name of that boss, but after all the hype from the beginning of the game to...that, it made me feel so disappointed, I had to put down the game for few hours.