Spoilers below. Proceed with caution.
Don't get me wrong, the other bosses are perfectly fine (with some minor flaws).
I agree on the points made on Asgore though. In traditional boss form, he takes what you've learned and flips it around so you have to call back on the entire game for what seems like the final stretch.
What really makes him stand out to me, however, is his personality and when he destroys the mercy button.
Asgore is a lovable boss in a way because despite the fact that everything hes ever loved has been taken away from him, he still puts other people's interests first. He shows regret and hesitation, and is generally an all around nice guy, which makes it that much harder to fight him and makes you angrier once smug-ass flowerpot, whom has been an annoyance all game, murders him in cold blood. With the mercy button, the game is breaking its own rules as you stated and forcing the player to fight, even on the pacifist route, so the weak-ish player is forced to struggle through a tense fight that makes it feel like the final boss, even though it isnt.
Asriel was good, but kind of annoying. Every time you died he went through the same dialogue over and over, doing the same moves. He felt more like a normal boss who does the same thing, like Muffet. If it werent for the fact that the fate of the world depended on the fight, the battle would be ultimately forgettable for me.
Flowey just seemed the most unnerving, and from my perspective, intimidating, powerful, and godlike qualify him as actual boss material.
The music for all 3 are amazing though.
Who am I kidding? The entire soundtrack is great.
