people actually think rafas not the best clay-player ever?
There is someone by the name of Borg. You may say "look at all those TMS titles Rafael has" but back in Borg's time, there were no such type of tournament as tournaments labeled and advertised as a MS event only started in 1990.
So truly the only comparable stat is the French Open wins.
OP's beliefs are very stupid, much like Start_da_fail's and FailSampras's remarks. It can be argued that upsets are easier to achieve on faster surfaces.
Either way, if there was a way to calculate who truly is the most dominant player on each of the surfaces and compare them to other players/surfaces, who cares. You don't get your name included in the discussion for GOAT merely by winning on one surface. Because on hardcourts and grass, Nadal isn't even in the top 10 whereareas Borg is arguably in the top of the discussion on more than just clay. And then Federer clearly is in the discussion for best player on grass/hardcourts, plus while there are obviously better players on clay than Federer, he can also be argued in the top 10 on that particular surface.