I liked pretty much all of the fiends (Even the Malboro), save the animorphogus (or something like that). It had a rather long name, and was found a lot in the Macalania Woods. It's a gel fiend, I believe, and I'm pretty sure it can take your health all the way down to one, regardless of your HP, with one hit. It took me forever to beat it. I had to use the Flarall Dressphere to do it.