How could everybody forget the Flumph!!! You just can't beat an intelligent floating frisbee with tentacles. And don't forget some of them had levels in cleric! It is so sad that they didn't make it into 3E. I'd want them as a player race.
Yes, tarrasque is good, but mine might have to be green slime, as I read a song from one of my brother's old polyhedron mags that was a Christmas song parody about green slime. A few years ago, I knew all the words