I love beholders, umber hulks, other cthuloid monsters, but my hands down favorite is the mindflayer. These psychic braineaters are perfect masterminds. I'm a fan of the underdark and when I think of those lightless depths, these bad boys and drow are the first things that come to mind. In addition, Spelljammer was a setting that I always enjoyed and these alien bastards were among the stars too. They are also a good way to introduce psionics into a campaign.
I also love that I've seen similiar looking aliens in other worlds, like Doctor Who's Ood and the Quarren of Star Wars.