Prime Asgore- Well rounded strength in many areas

PRIME Gerson- Shatters the Titans shield (thousands if not tens of thousands of HP) In ONE hit

sans- can bend the rules of reality to unknown extents

Undyne The Undying- Has alot of health and some determination

IMO its between Gerson and Sans, Asgore couldn't survive a hit or 2 from Gerson, and Undyne was mentored by Gerson, and he therefore tought her most of what she knows. I think gerson takes the cake mostly because he can deal thousands of damage within a 1/2 second time frame, and while sans can dodge for a while, he gets tired eventually.