Who do you think if they were more consistent and stayed injury free and lived upto their full potential would have ended up with a lot more slams than they do now(active). No Big three please (they already have many). I think Tsonga, del potro, kvitova.
I agree with most of the names mentioned, though "should" is always a tricky proposition I'll have to think more about WTA players. Tommy Haas (injuries) comes to mind for me, and going way back, I think Nastase, if equipped with a better temperament, had the talent to achieve a lot more.
How Safin only won 2 slams is beyond me. If he had his act together, should have cleaned up from 2000 to 2004, then another 2-3 slams during Fedal’s peak years (of which he got 1). Unfortunately he was too busy chasing the supermodels and clubbing. Resulting in poor conditioning, followed by injuries.
Off the top of my mind, of the best/most talented players that never have won a slam (that I've seen), Mecir, Rios, Nalbandian, and Monfils have to be towards the top of the list. The top underachievers in my opinion are Safin and Ivanisevic, who had the ability to win way more. Roddick was unlucky to have just one Slam title, given that he made five finals (2009 Wimbledon was probably the 2nd one he should have snagged).
He's undoubtedly already been mentioned, but my favourite player Nalbandian. He should have had 4 or 5, was IMO as good as or better than Fed talent wise, and was the most savage underachiever in tennis.
Del Potro should have won 4 or 5, Nalbandian should have won 3 or 4, Murray should have won at least 6, Safin should have won at least 7, Tsonga should have won 1 or 2 and so are Monfils and Gasquet...talking about 80's and 90's players, Chang should have absolutely won at least 5 or 6 times as many as he did, Courier should have won just a couple more, Kuerten should have won TWICE as many RG titles as he did if not more! Gerulaitis should have won a a couple more! On women's end, Novotna should have won at least 4 or 5 Wimbledons, Sabatini should have won at least 7 and i'm very serious! Mandlikova could have won just a couple more, Goolagong Cawley was exceptional in my book and should have went double digits! (nothing too over the top though, maybe in 10-12 slams range!), Justine Henin should have went double digits as well and achieved career grand slam at least once (better yet - twice...), Venus Williams shouldn't have collapsed in the beginning of this decade as well the way she did and should have kept winning slams along with sister (12+ slams range very likely!), Capriati twice as many as she did, Mauresmo between 6 and 8...and Hingis...boy is she the biggest underachiever of them all! The girl should have been like grand slam tally record holder amongst women by now! And i'm not even kidding!
Roddick, w/o a doubt. Should have had at least 1 Wimbledon (if not 2). Michael Chang as well (won 1 of 4 slam finals). Marat Safin certainly had the talent to win more. If he wasn't such a hedonist (womanizer & party guy), he would have had more. But he undoubtedly had fun and "did it his way".
Simona Halep has made it to 4 slam finals (1 AO & 3 RG). She should really have 2 RG titles under her belt already. She will likely reach another final in the next few years. She may or may not achieve another slam title.