In alphabetical order:
1. CatoblepasI always liked this monster, not sure why. I can see the warthog-like nature turning off some DMs, and it doesn’t fir in well with most enclosed (read: dungeon) environments, but I think the ultimate killer for this beast was its pure deadliness.
Among the Hisperies and Ethiopians is a well, that many men trow is the head of Nile, and there beside is a wild beast that hight Catoblefas, and hath a little body, and nice in all members, and a great head hanging always toward the earth, and else it were great noying to mankind. For all that see his eyen, should die anon, and the same kind hath the cockatrice...
—Bartholomaeus Anglicus,13th century CEDe proprietatibus rerum, book 18
Despite the limited chances of it raising its head, it basically amounts to a single DM roll spelling instant death for any character, regardless of level. (Novel thing that, in this 4e age!)
2. Intellect Devourer
My love of this one rides solely on a great Dave Trampier illustration and the idea of this thing clawing away while it mentally assaults you. The “headless” look of the monster adds to its overall weirdness, ass does its brain like “face.” If seen a few updates of this critter, so I guess for love for it is not a rare thing.
3. LeucrottaMy favorite on this list, hands down. These guys, to the best of my knowledge, appeared in just two adventures (Wandering Monster tables notwithstanding)—Temple of Elemental Evil and a side trek in Dungeon called “Their Master’s Voice.”
The combination of a jackal-like look, the weird mouth (which is based on the original myth), and their voice trickery is awesome. Mike Ferguson does a splendid update of these in Paizo’s Second Darkness adventure path (I adored the illustration Paizo used too); well done Mike!