Oh, another tough one. It’s a toss-up between beholders and illithid. They are both a lot of fun from a game stand point. And by fun I mean “AH! AH! AH! AH! RUN! RUN! RUN! RUN!”. Seriously though, one of the most fun adventures we have run was the Illithiad modules.
Although, the party did have a pet mimic for a while in Shackled City. He was fun. Guarded our house for us while we were out adventuring. Although, eventually we were gone too long and he got hungry and left. As I recall, the city guard had to put him down. Very sad….