Carrion Crawler. What? A creature that does 0 damage? Ah, but that's the beauty behind the crawler's appeal to me. How many times has your character been attacked 8 times, been hit 3-4 times, and forced you to save 3-4 times or be taken out of the fight?
The best carrion crawler miniature out there is the Citadel AD&D version - ADD72. The second best would be Otherworld's Carcass Scavengers (this one and this one). I have never been a fan of Heritage's Carrion Worm.
Honorable Mention for aberration is another zero damage critter - the Rust Monster!
Put one or more of each in a room and watch the entire party scream like girls. Hours of fun for sadistic DMs.