Juiblex is, by far, my favorite. As a kid, my brother ran me through In Search of the Unknown, the module that allowed DM's to populate some rooms themselves. Being very young and VERY new to the game, my brother populated one room with 6 Juiblexes! That has been a running joke for 30 years now.
I've always liked Demogorgon. He was THE dude back in the day. (1hp more than Asmodeus!) When MMII came out I just fell in love with Pazuzu. Lord of the skies above ALL of the Abyss (well, so he claims.) Tre cool.