You could probably narrow that down to the Klitschko's and Haye. Yes he would have problems with either Klit due in part to the size and ability of both of them. I certainly don't see him stopping Vitali. I think Haye is a better match up in style and size and probably closer to what Ali would have faced so this is the easier fight. In reality though any heavyweight great will have problems against top heavyweights from other era's, certainly in more modern times.
Ali would have cut through today's era like a white hot knife through butter. Even the best around,the Klitschko brothers,would be too slow to deal with Muhammad's speed and adaptability.
Wladimir would be a tough match, but Ali still wins like 7-5ish, dropping rounds early but dominating the latter half. I think Wlad would tire quicker fighting Ali with his feints and footwork. He wouldn't be able to control te pace like he usually does.
No disrespect to todays heavyweights but i think most would agree its been pretty poor for a few years now. These guys would be food and drink to Ali, or indeed, quite a few of the past champs from Louis right through to prime Holmes IMO
I feel he gets worn out and succumbs down the stretch to this heavier and more mature version of Briggs.
