A fighter at his absolute peak, against a lesser opponent, passing the eye test at his highest ebb, or clearly past his prime, but pulling off an unexpected upset against a seemingly much better opponent? Obvious example: Is Muhammad Ali more impressive to you beating Cleve Williams, or George Foreman?
I'll say a virtuoso performance at a fighter's zenith is more impressive than an upset when past prime. If you're so inclined you might not be impressed by the fighter they defeated and it's easy to argue how good or bad any given opponent was but I think a display of top level boxing talent is both inarguably skillful and a more impressive spectacle to watch.
The second principle actually proves how good someone is. Someone like Gerry Cooney looks a lot better fighting low level opposition than Ray Mercer for example, because he had a great punch selection and was a great finisher, which both make you look great against stationary targets, but I’d certainly pick Mercer to beat Cooney.
