WB's performance in Marineford is, still to this day, the greatest ever shown in the manga. He wasn't "only" an old man with cancer, he was also stabbed in the chest before even starting to fight. Even then, he still folded Akainu and AFTER THAT he beat Blackbeard, who only finished him with the help of his crew. That was monstrous and no character has shown anything close to it so far (maybe only Kaido in Onigashima is up there, but he was younger and faced lesser opponents IMO).
The series clearly portrays Whitebeard (and Kaido, to some extent) as a durability freak. In an 1v1, I think that Garp could win, but in a long, drawn-out battle like the Marineford War, WB has the advantage. I think we could compare WB to someone like George Foreman, he had an insane chin, a lot of heart and an incredibly powerful punch, even at an old age. But when he fights a genius like Muhammad Ali, he might be overwhelmed (it's worth remembering that Ali also aged worse, he was already showing signs of
dementia pugilistica in his late 30's, while Foreman managed to be a World Champion at 45 years old... so, not all people age the same).
Edit: I'm seeing now that it's Prime Garp you mentioned... he would beat old WB without a doubt.