Hard courts has a relatively short history as a big time surface. So judging just the last 35 years since it has come to prominence and had atleast 1 major event held on it:
1. Federer- isnt the best ever on either fast or slow hard courts, but has the best balance of dominance between the two.
2. Sampras- best ever on faster hard courts and very good on slower ones too.
3. Djokovic- best ever on slower hard courts and very good on faster ones. Will overtake Federer as overall hard court GOAT by end of career most likely.
4. Agassi- best ever along with Djokovic on slower hard courts, and excellent on faster ones. Was held back by Sampras denying him so many fast hard court titles, and not playing the Australian Open so often early in his career.
5. Connors- actually maybe should be higher considering he would almost certainly have won 6 or 7 U.S Opens on fast hard courts and been the overall fast hard court GOAT today had the U.S Open not been on clay for 3 years.
6. McEnroe- at his best some of the highest level of play on hard courts, but not consistent enough.
7. Lendl- I feel like this is low but not sure who to place him above of the ones above.
8. Nadal- super consistent on outdoor hard courts in prime years, and has won Australian Open, U.S Open, Olympics, and numerous Masters on them.
9. Murray- very consistently great results on hard courts in tough era.
10. Edberg- his 2 U.S Open performances are strong enough I put him above Becker and Courier.