I'd have to go with grass. Why? Because he's got a legitimate argument for being the greatest grass player of the Open Era. Roger's overall stats are better--one more Wimbledon, nine more titles, and a higher Elo (2547 vs. 2501)--and Novak is very close, maybe even #2 over Pete. But you can make a case that Pete was even better than both at his peak, because stats don't fully capture what happens on court, and of course the problem of stat comparisons across eras.
On hards, it is Novak and Roger, then everyone else. Pete's in the mix with Agassi, Nadal, and maybe Lendl for #3, but he's no higher than #3, and the gap between Roger at #2 and whoever is #3 is larger than the "grass gap."
On carpet, I don't know how you get past the overwhelming evidence that it was McEnroe above everyone else, then Lendl and Becker and maybe Connors, then Pete's in the mix with Laver and Borg for the 5th spot. But he's no higher than #5. Plus, Pete's carpet Elo (2407) is significantly lower than the other guys. And note his 4-7 record vs. Becker on carpet, and 1-2 vs Lendl on carpet. He never faced anything close to peak Connors or McEnroe on carpet.
Of course all of that is relative to other players. Going purely on Pete alone, from what I've seen, I'd still give the edge to grass, but I didn't see much of him on carpet. Part of that is his competitive spirit, and that he brought his best to Wimbledon and Slams in a way that he didn't at lesser tournaments.