Beholder, albeit for slightly different reasons.
From a game standpoint, beholders are just rocking monsters. They offer a ton of coolness. They are one of the few monsters that isn't underpowered (relative to the party) by a concept taken from mythology -- I mean, unicorns in mythology are powerful creatures, but really, their only power is being good and pure and knowing when things are evil, so in D&D, they get shafted. Detect Evil, Cure Light Wounds, and that random teleportation thing. Feh. In mythology, they're hugely powerful, but in D&D, they're lame.
Beholders, though, don't have any mytholgy to draw from, so they can just be effin' NASTY. And that's great.
Also: beholders offer the ability to challenge the party across a wide range of levels right out of the book. A 5th-level party has some chance to defeat a beholder if they can get the drop on it -- it doesn't have THAT many hit points, and it's AC isn't spectacular. I think I threw my first beholder at the party when they were 9th level, and it was an ugly fight (because the party wasn't surprising it), but they got lucky, and felt great.
But a 15th-level party can STILL be challenged by a beholder, because it's such a glass cannon. Sure, it only lasts a round if the party is ready for it, but if it gets the drop on the party, it can wipe out half the team before they know what's up.
C'mon... who here hasn't had the party walk into an enormous dark cavern, their pitiful light spells flickering against the dooming darkness... and then the lights go out, every spell on the party suddenly drops, and a moment later, the unarmored party wizard is hit by a 375-lb rock for 13d6 damage (Most a beholder can lift with telekinesis, and can throw it as a ranged attack modified by Int -- and against the AC of magically bereft wizard, that's a pretty easy hit for a boatload of damage.) Even a 20th-level wizard can get hurt by that.
And even when my 20th-level party took on the forces of hell themselves, even when they were fighting advanced half-fiend gorgons and advanced half-fiend manticores (with unlimited tail spikes per day and boosted Dex instead of Str), what was the party worried about? The 9 beholders floating overhead whacking people with bunches of ranged touch attacks per round.
Beholders, man. It's all about the beholders. People say that D&D has its own mythology, which is different from normal mythology and separate, and which creates problems in making things interesting (eg, how much work do you have to do to make giants as cool and frightening as they are in Norse legends, or to make the Fey as mysterious and deadly as they are in Celtic myth?). But beholders are perfectly in line with D&D mythology. They're just absolutely rockingly perfect.