I go with Krajicek as the underachiever (relatively). He had arguably the biggest "A game" on tour for awhile but managed just 1 slam final. Watching him at 18-19 was like watching Milos Raonic with a ground game and volley's. He seemed destined for greatness (rather then just very goodness)
Stich, Cash, & Goran had multiple slam runs. They got unlucky not to collect a few more rings, but they were in the mix.