Clerc, I think he is one of the best players who never won a slam.
I would agree. He was a fantastic talent but I think his head is what kept him from winning a Slam. Sadly, he had a tendency to choke in many of the the big moments. He seriously underperformed in the Slams. Amazingly, Clerc never even reached a Slam final. For a guy who won as many tournaments as he did and who had as great of an all around game as he did, that's pretty shocking. I know he reached the semifinals of the French Open in 1982 (losing to a young Wilander in 4 sets) but other than that I'm not sure he reached another semifinal of a Slam. Another choke situation for Clerc was his match against Borg in Monte Carlo in 1983 which was biled as Borg's last tournament. Clerc was so nervous he lost to 6-1, 6-3 to Borg who was a shadow of his previous self (Borg easily to a very young, unpolished Henri Leconte in the next round) while Clerc was in his prime.
But if you check his head-to-head results against some of the best of his era his record is generally damn good. He was 7-7 against Lendl, 3-1 over Connors, and 5-5 against Johnny Mac. That's impressive stuff. Makes his lack of success in the Slams even more curious.