While the Gelatinous Cube has its merits, I chose to go with an Aberrant rather than an ooze. While the Mind Flayer is certainly one of my most favorite aberrations, there is one that I like just slightly more:
Beholders are nasty and cruel creatures, and their eyes shoot all sorts of magical rays. When that big floating mass appears, the players know that it is business time, as a Beholder can easily turn a simple fight into a terrifying life-or-death struggle.
The one time I was able to have my players fight a beholder was pretty awesome. I don’t remember much of the fight, but I do remember that it was tons of fun and that the warlock of our group looted a Beholder Lens off of the beholder’s body. The beholder was using the lens to split and empower its ray attacks. I told the player that he could use it maybe once, and he did – when he managed to slay Asmodeus on one hit.