I'm generally wary of ranking anyone from before the earlier decades when I've seen only a handful of his/her matches at most and we don't have the usual stats to go by, hence my earlier "since the '90s" caveat.
But since you've brought him up I do think Lendl is up there. In his prime Ivan almost certainly approached if not surpassed Courier's 64.3% of games won, and we know he had the firepower, patience and (almost in '81 but later on) endurance to trouble even Borg in a RG final.
Having said that I think Kuerten's ceiling was even higher, in fact probably the highest ever if we're talking only one match in full throttle. Power is pretty much a wash between the two: Guga definitely went for broke a lot more, which is why he remains the most exciting CCer I've seen, but while BH probably goes to him in any setting Lendl bests almost anyone else when he's ripping his FH. (BTW when it comes to FHs in both top-end pace and weight of shot I'd say Ivan is matched only by Pete and Gonzo and surpassed only by Delpo in bulldozer mode.) Endurance is another nonfactor as I don't remember seeing Guga ever get winded. Where Guga holds the decisive edge, I think, is in (you guessed it) movement. The guy simply had more natural feel for dirt, and Guga's long limbs gave him even more deceptively good court coverage which probably overrides Ivan's edge in patience and shot tolerance. I'd also say Guga's serve was a hair or two better if only by virtue of his height. And possibly more clutch! (Or maybe I'm too bowled over by his serving in the '00 YEC final.)
BTW we shouldn't forget Wilander. The guy may not be able to bring the same heat as these two but he was arguably the craftiest of all ATGs and I can definitely see him giving his fellow multi-RG champs and even Borg and Nadal fits with his unpredictable serving and net game, good ol' dinks and slices, and the best defensive lob I've ever seen (at least among men, with all due respect to Chang). They say the 3-timer would be overpowered in today's game and get spanked by Borg and Rafa who would dominate him in baseline exchanges and pass him left and right if he dared come to the net. Sorry, I don't buy it.
Having said that I still would put Ivan and Guga over Mats and also Bruguera. Speaking of whom I must say I doubt Ferrero would win the FH battle with his compatriot when Sergei could probably match Rafa in sheer spin with poly (though perhaps not quite in weight of shot). Also get this: JCF's CC peak was arguably '01, when he won Rome, 40.3% of his return games (the only time he cracked the magic 40% mark) and 60.8% of his overall games (another career best)... only to get destroyed by Kuerten at RG. Granted this was an en-fuego Guga, but still.
But even if JCF could somehow overpower Bruguera at times this is clay we're talking about, where consistency usually wins over aggression (unless you happen to be a physical freak like Guga) or flashes of brilliance in the long run. The '93 final was but one example among many, and while there are exceptions like Fed's win over Novak in the '11 SF or Verkerk's almost entire '03 run we all know what happened in the end. Still like Sergei's chances against almost anyone, though there would be some days when any of these champs catch fire and make it difficult for their peers including Borg and Rafa.
And since I've bloviated this much I'll give my all-time CC list a try. Here goes, since the mid-'70s only (don't feel comfortable going before that):
1) Nadal
2) Borg
3) Lendl and Kuerten (tie)
5) Wilander
6) Bruguera
7) Courier
8) Djokovic
9) Vilas*, Muster and Federer (tie)
12) Agassi
13) Panatta and Wawrinka (tie)
15) Chang
16) Kafelnikov and Costa (tie)
18) Gómez and Moyá (tie)
20) Noah
21) Gaudio
Notes/explanations:
- Think Sergei might be up there with Mats, but 3 (plus 2 more finals) beats 2.
- I actually think Novak vs. Fed prime for prime would be pretty close, but I like Djoko's rock-solid consistency better against the rest of the field, hence his higher ranking.
- Vilas is the trickiest pick here and could easily be placed higher or lower. That said I'm giving the Argie much benefit of the doubt cuz it's very arguable whether he bags a FO without WTT, but his GW% on his '77 run was up there and he gets extra credit for his legit USO title that same year (but only a little cuz I consider terre battue and Har-Tru fundamentally different surfaces).
- Both Panatta and Wawrinka aren't typical dirtballers by any means, but Adriano gets a big boost for being the only one to dethrone prime Borg at RG (his other W in '73 is fairly meaningless) while I think Stan's firepower would make him dangerous against almost anyone.
- Not only does Chang have an extra final over all of his lower-ranked peers, his bigger bag of tricks and almost pathological resolve would see him through many a match against superior opponents on paper, as we saw during his '89 run and in the start of his '95 final vs. Muster.
- Kafelnikov is another atypical FO champ, but his '96 victory with the loss of only 1 set was impressive and while we can talk matchups nobody else troubled prime Guga as much on his later runs.
- Gomez is notorious for being the chief beneficiary of Lendl's absence from the '90 FO, but his win over Agassi was well deserved and his FH and Moya's push them over Gaudio who may be more of a dirtballer but lacks a big weapon of his own.
- Noah's serious technical deficiencies which later got exposed (kudos to Datacipher for his astute analysis) probably should earn him a co-bottom spot with Gaudio, but he does have a whopping 4 extra QFs. Plus that service motion!
Heh I shoulda figured. In my defense Bruguera's ultra-quick time lapse between points reminds me of the old-timers, so 3 hrs was somewhat plausible.
And underappreciated, not underrated which implies a fair amount of discussion to begin with. Their '93 tussle simply isn't talked about much cuz it lacks a marquee name... although it was most likely better than any of the FO finals since and quite possibly the very best of the Open era! (I happened to watch this and the infamous '84 Lendl-Mac final not far apart from each other years back and the '93 marathon struck me as superior in almost everything but variety of styles and historic value.)
Yes and I think those two RG Ws for Bruguera clinches it. Jim was an all-timer whose game was well suited for clay, whereas Sergei was a natural dirtballer who despite his well-known laziness put it together for his two runs.