Which notable players in chess history successfully used dubious openings? I am reminded of a famous game between Karpov and Tony Miles where Miles played 1...a6 against Karpov's 1. e4 and still won the game. 

     [FEN "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1"]
     [White "Anatoly Karpov"]
     [Black "Tony Miles"]

     1...a6?!


Which other players in chess history have used dubious openings successfully? Which were the openings? 

NOTE: For the purpose of this question, the player (or the opponent) must either have been a world champion (like Karpov in this game) at some point or at least a strong contender for the world title during his time. Also, the openings must have been considered to be dubious during their time.
If you really mean "dubious", then no one really fits this description since Steinitz, who liked to, for example, go for walks with his King when playing the King's Gambit as White. But people didn't really know better back then.

If you're willing to relax "dubious" to "offbeat", the first player that comes to mind is Bent Larsen, one of the strongest players in the world in the 1960s and 1970s. He regularly played 1.f4 and 1.b3 (often known now as Larsen's Opening), and also helped revive the Bishop's Opening and the Philidor Defense. None of these were regarded particularly seriously by top-level players at the time that he adopted them.

Before that, players like Réti and Nimzowitsch developed hypermodern theory, in which players tried to control the center from afar instead of occupying it with their pawns. Some players regarded this strategy as dubious but it shortly became clear that it was perfectly legitimate.