Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-13682699-20171112024821

When it comes to how hard a boss was for an individual, each person will have a different experience and thus their respective thoughts about the experience of each encounter is simply an opinion.

However, when it comes to how powerful someone ACTUALLY is in this game, there is a quantifiable number for it, which is the persons ATK and DEF and to a lesser extent LV/EXP/HP; because of this, there is an order in which the monsters of the Underground can be ranked.

Most Powerful: Asgore/Toriel/Undyne/Mettaton

Those four are the undoubtedly the most powerful monsters in game based on their actual stats without holding back.

Of the rest of the main characters: Papyrus, Sans, Flowey, and Alphys, Alphys is the only one that we have no actual physical data on and is not actually trained in combat, and as such is therefore disqualified which leaves Papyrus and Sans. Papyrus is next strongest of the main cast, having solid stats and being praised by Undyne as actually being really tough. If he wasn't so nice, he would have definitely had a spot in the Royal Guard.

Flowey has 19 ATK minimum according to your only encounter with him in his regular form and is technically the most powerful due to his ability to RESET and has mentioned killing everyone in the Underground before (including Sans/Toriel/Asgore etc., EVERYONE) using such an ability, however for sake of the actual stats, he is right below Papyrus in terms of power.

Of course that leaves Sans. Sans is technically the weakest of every monster in the Underground by stats, but makes up for it with his meta knowledge as well as his Karmic Retribution. Being a sentry in Snowdin, he is also trained in combat. As everyone knows of his challenging boss fight, it would be pretty easy to say that if Karmic Retribution wasn't such a situational power, and if he had stamina to last longer than he currently does, that he could possibly be among the ranks of most powerful right alongside Undyne. However that is not the case, and because of that, he ultimately should fall below the most powerful group in any list regardless of how tough his battle was.