Aboleth - if you want a good aquatic mastermind, look no further.
Balhannoth - invisible and teleport. Eh. Just use a lich.
Beholders (beholder, death kiss, gauth, gazer, spectator, etc.) - overdone and overrated. I've replaced them all with blast spores in my game.
Berbalang - Now we're talking.
Choker - it has tentacles AND knows how to use them.
Chuul - eh. There are better options for "weird crustacean" out there.
Cloaker - terrorizing characters for generations
Illithid (elder brain, intellect devourer, mind flayer, mindwitness, neolithid, ulitharid, etc.) - get your cheesy sci-fi out of my campaign, you wierdos.
Flumph - you know this was created as a joke, right?
Gibbering Mouther - brilliant in the hands of a good DM
Grell - your players will be too busy laughing at it to fight effectively
Morkoth - it's like they aren't even trying to be original anymore.
Neogi - intelligent spider people from outer space. And "from outer space" is always code for "psionics lol".
Nothic - fun to fight at low levels.
Slaad (Blue, Death, Gray, Green, Red, etc.) - "but it has an ovipositor!" Pbbbth.
Star Spawn - Ever get eaten by a grue? Now's your chance!