I wouldn't consider JJ, Dr Salvador or the Bella Sisters even mini bosses if you don't put the Regenerators/Iron Maidens as mini bosses also. Afterall, the Regenerators/Iron Maidens are far tougher to kill.
Every boss is easy to kill if you use the rocket launcher. But assuming you don't have the rocket launcher, Verdugo is by far the toughest boss to kill. But I just realised he isn't an option on this survey!!?? You also forgot to list U3/It (the serpent ugly thing, sort of half human...I don't know how else to describe it), who you fight just before you fight Krauser.
So since I can't vote for Verdugo...I'll vote Mendez. I just found him very awkward to fight...his attacks can be devastatingly quick (and on pro, almost always instant kills) when he is in his second form, and when he's in his second form he's so agile which can make shooting him far from easy..