Don't know of any losses in important matches. By which I mean nationals more or less. I think he has two losses (at least) to Milan and there may have been many more since they probaly met a couple of times every year but I know of no losses in the nationals.
Stevenson would have certainly made things more interesting. Ali struggled against less talented opponents so maybe. I lean a bit more towards Ali because I've noticed a tendency for many Cuban fighters who turn professional but don't have the self discipline to maximize their talent, however, there are exceptions to that trend.
Years ago there was a great article by I believe Scully about Stevenson's rivalry with and the guy who had his number, Vysotsky.