That’s a tough one, but I’d probably have to say the girallons. Nothing more terrifying than a 4 armed ape that can literally rip party members in half if they get a good grip. We had one encounter where just a couple of the things really beat the snot out of our party. It wasn’t fun.
Second favorite are beholders. Always a tough battle whenever you encounter one just because there are so many different types of attacks to try and defend yourself against, especially with the anti-magic cone from their main eye punking your abilities to do much damage to them.