Well, he beat the living hell out of Archie Moore, who was a darn good fighter, and looked absolutely relentless and unstoppable in that fight, with brutal nonstop power shot after power shot at a breakneck pace.
my choice is charles 2, partly because it is the best performance against him that ive seen extended footage of and partly because it shows him overcome real difficulty. I have the feeling charles 1 would be a better answer, but i've only seen highlights of that fight.
That Moore fight is something. Moore an all time great puncher nails rocky with counter after counter and Rocky just keeps coming (like he always did). The sixth round the last minute is amazing. You can choose a few from Rocky's catalog but that Moore fight is something to view.