Title says it all! GO!

The bananasaurus. I'm a lance user and since he is not standing on 2 legs the lance charge will hit him for the whole duration of it which is extremely satisfying.

i have the most fun with the rathian and rathalos, but i still haven't fought the toughest monsters yet.
 
so my least favorite: those damn rheno-whatevers (dino looking things) are the lamest monsters ever. so annoying and dumpy looking but mess with me everytime i fight rathalos.

Deviljho... It's effin scary when one appears and rampages throughout the stage, taking the spotlight from the monster the group is focused on. I mean hell, the game throws in his battle theme whenever he appears, ignoring whatever else you were hunting. (He can appear in almost any high-rank quest at random, and the quests directly against him appear after HR50 )
 
He also hits real hard... real, real hard.