Beholder 26 Looks like Beholder wins which is fine but...
Gelatinous Cube 20 But this guy/gal is just great and enters at a much lower level so maybe sees more heart-thumping, terror-fighting action.
Mimic 10
Mind Flayer/Illithid 19
Owlbear 11
Purple Worm 9
Rust Monster 9