Board Thread:General Discussion/@comment-37750220-20181214063221/@comment-34974603-20190731055909

It honestly depends. Undyne the Undying was more difficult for me, but that's because I'm better at platformers than I am at bullet hells. However, it is the widespread belief that yes; Sans is the most difficult enemy in the game.

Although if we were debating "strongest" and not "hardest" then I'd say Asriel would be the strongest. Dude literally becomes God. Second would be Omega Flowey until he loses control of the SOULs. Up until he loses control over the SIx SOULs he literally spends his entire boss fight screwing with you, even though he could one-shot you if he wanted. The third strongest would be either Undyne in her Undying form or Sans. Although, there's also the theory that KARMA only affects you if you have a high LV. In that case, Sans wouldn't be all that powerful if you fought him at LV 1 because you'd have your invincibility frames back. But that's up for debate. And there's also the fact that Sans can only dodge you approximately 24 times. Remember that your attacks basically involve you running up to him with a knife. Imagine if someone like Undyne fought him. He'd need to dodge 24 flying spears in one attack. Of course, he has teleportation, but Undyne would probably still outlast him. Overall, considering the stat and HP difference, I believe Sans would probably lose if he fought Undyne.