@Moxie, haha, of course.
Just to be fair to Grigor, while he may still be an underachiever, winning two big titles last year somewhat changed that. He now has more big titles than Tomas Berdych (1), David Ferrer (1), Milos Raonic (0), and Kei Nishikori (0), and as many as Tsonga (2), Zverev (2) and Cilic (2).
To put it another way, he has equal or more big titles than all but five players on tour (Big Four + Stan).
OK, no Slams. But he's not done and still could slip one in before he's done, or at least a couple more Masters titles.
For me the biggest underachievers are probably Tsonga--who really should have won at least one Slam by now, but he's French--and Nishikori, who while not having the big-time firepower of most Slam winners, has just been so rock solid to not at least have won some Masters. I think you can make an argument than Kei is the best player in the Open Era not to have won a big title.
In fact--I had to double-check this--according to Ultimate Tennis Statistics' GOAT formula, he is:
http://www.ultimatetennisstatistics.com/record?recordId=BestPlayerThatNeverWonBigTitle