I'm going to cheat a little bit and paste what I've written on another board:
Mine is the Gelatinous Cube. Maybe it's because they are tidy, equal-sided polyhedrons (I also love modrons) and it appeals to my sense of pattern and order. Maybe it's because they are absurd. Maybe it's because it is so obviously a D&D-ism, a monster that lives in a place neatly defined by 5' X 5' squares that often needs to cleaned of loose change, and bodies, lying about. Maybe it's because they are weird in a near-Cthulhu sense, being composed mostly of a living, but mindless, jelly-like substance that is corrosive and near invisible in the dark. Maybe it's because the image of a slowly-moving haze filled with loose objects and bones fills me with dread.