Tough call.
Stu Hart and his "Dungeon" are credited with producing the likes of Dynamite Kid, Davey Boy, Benoit, Lance Storm, etc. The problem I have with that is that most of those guys had prior and/or additional training elsewhere. They may have gotten their North American start (first exposure) in Stampede, but that's all the Harts can honestly take credit for in many cases.
Verne Gagne's camp turned out some fantastic talent. Verne had guys like Billy Robinson and Iron Sheik up there training the freshmen. Some guys, like Steamboat, had an amateur background, which Verne loved. Others, like Flair, were a huge work-in-progress. From what I've heard, Sheik did most of the formal grappling training. Robinson would come down only occasionally to "stretch" trainees; more to satisfy his own ego. Verne made those trainees, work, though. You can find old footage of some of those brutal sessions that were designed to weed out the week and create superior, legitimate athletes.
Karl Gotch trained both of Boris Malenko's boys, both of whom have outstanding reputations as legitimate shooters. Gotch also took his style (honed from what he learned on several different continents) to Japan, where he was responsible for teaming with Antonio Inoki to literally revolutionize the pro style over there. It started in NJPW, but soon expanded to Baba's All Japan and others.
I would have to say that Gotch gets my vote based on those facts.
