Borg - ok.
Cochet, Lacoste - hardly enough competition in the 20's. Closed to pro players, Von Cramm - weak competition again.
Lendl, Nadal, Kuerten - ok.
Rosewall, Segura, Trabert, Santana, Nuslein, Budge, Gimeno, Laver... Divided
in their pro/amateur status.
In other words, lets compare the Open era players, as it's easier.
Nada,Borg, Wilander, Lendl, Kuerten - give you that.
Muster, Ferrero, Bruguera, Villas, Courier.
Villas has 1 title - Borg was not playing that year, and he won it against Gottfried.(won his US open, played on clay that year against Connors, who isn't great on clay either. In other words, if Nadal doesn't play at RG, I'm sure who's going to be #1 favourite).
Bruguera - 3 finals(total), won against COurier and Berasategui. Only 2 MS on clay.
Muster - 1 title, def. Chang, no other final at RG. 7 MS on clay.
Courier - 2 MS on clay, alike Bruguera - 3 finals, 2 titles, beat Agassi and Korda.
Ferrero? 2 finals, 1 win against Verkerk, 0-3 against Federer on clay, hadn't taken set off him.
Do you really think Courier, Bruguera, Muster, Ferrero have a bigger chance to beat prime Nadal at RG?
So top 6 clay courter in open era(so far) is a fair shot.