Board Thread:General Discussion/@comment-38589433-20191206210117/@comment-39394247-20191211170456

It's because of KR and his ability to teleport that Sans is usually the hardest boss in the game (Some claim Undyne the Undying is harder). I'd say Sans is in the middle of the power ranking, ahead of most normal monsters, but behind Muffet, Mad Dummy, Alphys, etc.