I'd say Muster being the 3rd best clay courter is about right. I'd put Borg and Nadal as being #1 and #2, respectively. Vilas #4. #5 is a toss-up: Kuerten? Lendl? Wilander? Bruguera? Courier?
ok, now I'll deal with Lendl(though I can't believe anyone could think Muster is a better claycourter than Lendl, though I can sort of understand why one would think Muster was better than Guga. A well past his prime Lendl beat Muster in straight sets at Barcelona '92)
1986-
at this point in his career Lendl has already racked up many minor clay titles, Muster style. Now he's more concerned with being #1 & winning Wimbledon.
Still he's not neglecting his best surface, clay:
He plays only 3 clay events, winning 2-Rome & Roland Garros-the 2 biggest clay events on tour at the time, losing only 1 set at the French, 2 in Rome.
He leaves the Emilio Sanchez's & Perez-Roldans of the tour to get some meaningless stat like "most clay court wins" for 1986, which I guess is important under your criteria. makes the finals of Wimbledon, by the way.
1987-
plays 3 clay events, wins 2-Hamburg & the French. What I wrote for 1986 summs this year up as well. Again makes finals of Wimbledon.
going back to 1985-
plays 3 clay events, wins 2-Forest Hills, Monte Carlo(guy won a lot of the big claycourt titles it seems) finals at the French
so to summarize:
wins hamburg twice
wins rome twice
wins monte carlo twice
wins the french 3 times
9 important clay titles for Lendl. 6 for Muster.
But Muster has 40 total clay titles, while Lendl has "only 28"(Borg has 29)
and this was all done while not skipping the grass/hardcourt season(& doing quite well in them), like the Moo Man so often did.
too bad Lendl didn't concentrate on that clay court win streak stuff, not sure he loses sleep over not being on some list with nadal, vilas, & muster.
Guess he should have concentrated more on playing those summer clay events, instead of preparing for winning 3 US Opens, during the 80s.
maybe I'll do Wilander next...