The beholder: perhaps D&D's most iconic monster. Also a nightmare to balance.
The basic chassis is surprisingly weak for a CR 13 monster. Decent but not great ability scores that fail to include one specific focus, a slow fly speed, some natural armor, and eleven wonderful aberration RHD (aren't we happy?).
Then again, you don't play beholders for the stats, you play them for the ability to cause petrifying, disintegrating, and necrotizing death at 100 feet. And let's be honest: the beholder is very good at that. Your eye rays include save-or-dies, charms, direct damage, control, some out-of-combat abilities... A well-rounded set of powers all around.
Less interesting is the beholder's antimagic cone. If used at range, all it does is take both you and your foe out of the battle, and if used in melee the weird shape allows people to just walk around you and stab you.
A problem I ran into was beholder support. At-will disjunction for two feats or a prestige class whose name has become a synonym for brokenness are nothing to scoff at, and I can't help but wonder if there is a way to actually balance this.
In the end, I think I'm going with ECL 15 here. You sacrifice most of your build, get much in return, and (hopefully) end up with a balanced character.
As always, your feedback is requested.