Whoa whoa whoa..!! I vote for Red Demon
Curly Brace is one of my favorite characters... but as a boss her AI is way toooo limited. The first time I fought her I DID die!! But, once you figure out how to fight her she's too easy...
Balrog and Misery in my opinion should be classified as Major Bosses
Misery especially is a pretty tough boss... [especially when you are new to her] Which made it hard to choose...
Red Demon? Well, unlike the other bosses when I reached the unexpected Red Demon battle, I had played Last Cave a lot of times... and reaching Red Demon was a critical point..... I didn't want to die!! Not now!!! [I had NO life left also... so one hit and I was doomed.] It was just an awesome battle, since you had to work your way to him. And turning to stone??? How cool is that
Then later on you discover he has a whole backstory..!! WHOA!!
By himself he's just a pretty simple minior boss... but add in that stuff and he rocks
[Frenzied Misery is pretty awesome, but I never paid her any attention and still won the undead core battle... and Curly is a mother to those 4 Mimiga, which she defends with her life! Far from a lipstick toting Chacobo we know...]
EDIT: Yeah I knew SP was just messing