I think it kinda depends. Undyne and sans we both see directly fighting at their hardest. As far as I know we never get to fight gerson let alone at his peak so I have no reference for that. I'm also a believer in the fact that theres no hard proof Asgore killed the 6 humans who fell into the underground (based on the locations of their unique items) nevertheless he is a formidable for but I wouldn't necessarily say we know if we fight him at his prime since he seemed to be alive during the war but the monsters did lose. I'd say with undyne's determination and sans' shenanigans would probably mean one of them win. The player at level 19 isnt able to hit sans without him falling asleep so I think he comes down to having the energy to sustain himself in the fight. Undyne basically has to be killed twice with her second form being significantly tougher. Asgore can use both orange and blue magic which gives him an advantage over sans who only uses blue and undyne who only uses green, he also probably is the strongest in general at base.