I believe that Stich and Gerulaitis are the only one time winners on the men's side that reached the semi-final stage or better at all 4 slams. Plus Stich of course won the YEC on carpet in 1993, beating Sampras at his peak (during the height of his dominance) in the final. In terms of the 'eye test', I always thought that he had the best all-round game out of all of those players (and his ability to transition between playing a serve and volley game and from the baseline was notable). His record at the biggest events on all surfaces further hammers that home IMO. Of course the 'lesser status' of the Australian Open title for quite a few years was a factor for a period. For example Orantes only entered the Australian Open once during his career, and reached the quarter-finals there. Had Del Potro not been so injury prone he surely would have reached the last 4 in Melbourne. However Stich played in more polarised conditions than he did.
On the subject of Gerulaitis, he was by far my favourite one time winner, and was a joy to watch, playing in numerous memorable matches on all surfaces. But his 2nd serve was a glaring weakness, and he had trouble hitting effective topspin backhands slicing it most of the time. So Stich was definitely more talented IMO. Gerulaitis was unquestionably the biggest 'star' out of all of the one time winners.
Out of the players with one handed backhands, Stich had one of the best backhand returns that I've ever seen (alongside his compatriot Becker). The backhand overhead is IMO the most difficult shot in tennis, but for Stich it was a speciality.
On the subject of Orantes, he was also an incredibly talented player with tremendous variety, craft and touch. His offensive forehand topspin lob was a thing of beauty. He is unquestionably the best player (in terms of clay court ability) never to win the RG title in the open era in my opinion.
