Stormonu
NeoGrognard
In 3e...none that I am aware of. They all lose out in the action economy (being outclassed 5 actions to 1 sucks).
Maybe if you had your beholder hover directly above the party and divided them into 4 quadrants, then it might be a viable solo threat (in that it could actually use all 10 eye rays each turn (as a free action to boot!)![]()
Dragons will do it in 3E. I had a Green dragon named Verdenthorax that defeated a party twice (2 PCs & a charmed ettin tried to take it on first and ran, they came back with the whole party and I proceeded to TPK them with the dragon - unintentionally, but you get what you get when you provoke the damn thing after it retreats to heal). If I remember right, they were about 6th level and the dragon was 4 CRs above them.