In my opinion Ali should be favored over anyone who ever fought, and is the greatest heavyweight of all time. That being said, styles make fights and even the Great Ali of 65 could have an off night, so with that in mind I give the following fighters an almost even chance of scoring the upset:
1. Larry Holmes: Battle of great jabs, master boxers, and genius level boxing IQs. Difficult to picture this one because Larry became who he was through sparring Ali, but this all pure fantasy anyways. I think prime vs prime, best night vs best night, this would be Ali's toughest test.
2. Joe Frazier: Joe had a difficult style for Ali, and had a "thing" against him. Much like Ken Norton, this style would always trouble Ali.
Ken Norton: similar to Frazier, Norton was a volume guy who excelled vs boxers while struggling with punchers.
3. Riddick Bowe: Say what you will about big daddy, but he was a huge, complete fighter the night he beat Evander the first time. This versions jab, size, inside fighting, and power may have been more of a threat than Holmes'...
4. Wlad Klitschko: Pure size disadvantage and bad style match up. I think Ali would have worn him down, especially his prime version, but that jab and fluid boxing mixed with freakish size could've thrown him off his game enough to allow the upset.