Trolls, beholders, Lich's, Mindflayers, zombies, mummies, skeletons, ghosts, vampires, werewolves, dragons. The standard D&D rogue's gallery. They are used so often, that most players will be familiar with them and their abilities.
But D&D has tons of monsters, many of which probably don't see a whole lot of use. I prefer to use monsters that my players may not be familiar with, or I alter an existing monster to make it less predictable. And when I do use a classic monster that is well known, I don't make a mystery out of it. This game of guess-the-weakness just isn't very compelling to a whole lot of veteran players and DM's alike.
