Board Thread:General Discussion/@comment-28032065-20160419130638/@comment-24828059-20160422182435

Honestly, even though Undyne the Undying and Sans are not the easiest bosses around, they are nowhere as hard as many other bosses.

Give Space Mama from Rayman 1 (a platformer game) a try and Sans will look like kindergarten material.

Also, this goes without saying, but the Extra bosses from Touhou games. In my opinion, the three hardest bosses from the series would be

Note that I've never played Legacy of Lunatic Kingdom or any game on Lunatic.
 * Marisa Kirisame (Great Fairy Wars)
 * Koishi Komeiji (Subterranean Animism)
 * Yukari Yakumo (Perfect Cherry Blossom)