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Aboleth 19
Adult/Ancient Chromatic (Evil) Dragon 22 Adult/Ancient Metallic (Good) Dragon 2 - this would be my third 'assist' if someone kills this off before it gets another upvote
Beholder 24
Death Tyrant 22
Dracolich 24
Elder Brain 20
Kraken 23
Lich 26
Mummy Lord 17 Vampire 9 - maybe not the most powerful monster on here (until you give them a pile of levels and abilities) but nothing is more legendary