What is the hardest monster in monster hunter tri

jaggi.

Erm no. I find diablos pretty hard, barioth is badass but alatreon wins the show for hardest boss.Actually all of them are really hard so tis game sucks.

Depending on what weapons you use the easiest i would say is a Jaggi or a Baggi, the rest are just annoying to fight barroth the most i think with the continuous charging and shaking of the mud. alatreon is ok when you get the hang of it and if you use a lance with guard +2 you would block most of his attacks