Just to highlight Lendl's case for being the best of the rest after Nadal and Borg.
He won 28 clay court titles and at least 2 titles at each of the 5 most prestigious tournaments available to him on the surface; RG (84, 86 and 87), Monte-Carlo (85 and 88 ), Rome (86 and 88 ), Hamburg (87 and 89) and Barcelona (80 and 81).
Lendl was also one of the greatest players of all time on both carpet and hard courts, and hence entered and won a lot of big tournaments on those surfaces. Thus it wasn't as if he adopted a clay court dominated schedule and padded out his record by racking up a lot of small, minor titles on the surface as well.
He has winning head to heads against Wilander (6-4) and Vilas (7-5) on clay, and in 1992 he beat both Muster (the Monte-Carlo champion that year) and Bruguera (the 1991 Monte-Carlo champion) on the surface.
Gomez (who lost to Lendl 4 times at RG in the 80s) thanked him for skipping RG in 1990, which goes to higlight his clay court reputation.
Wilander is not too far behind Lendl, but Lendl's overall clay court record is slightly better. None of the two time or one time RG champions are in his league.