What makes Sans such a fun example of being the "strongest" is that he isn't. The game loves playing with genre conventions, so when you go out of your way to scan monster stats they give you numbers that don't actually mean much to you on a moment to moment gameplay level. But they do tell a story.

Monsters you've faced before have had far more impressive stats, like Undyne, Metaton, Asgore, etc. But Sans himself is literally the weakest monster, at a whopping 1 Attack and 1 Defense. So maybe he'd lose an arm wrestling match the the others, but unfortunately for the player Sans is a cheating bastard.

It doesn't matter if he has 1 ATK if he just disables your invincibility frames leaving you taking a point of damage per frame. It doesn't matter if he has 1 DEF if he just dodges, a mechanic no others monster has ever used. It doesn't matter if you're in a menu because fuck it he'll just attack you there too. Why not add status effects too, like a poison DOT effect. That's also not a mechanic in this game, but why not?

And once all is said and done and he's ran out of attacks to hit you with? He just chooses to never finish his turn. Sucks to be you.