Look at OBW in that fight. Does it not appear that he is as emotional as Anakin? Anakin is emotional in that he wants to kill his old friend. He really want to do that. OBW doesn't want to do it. He will do what he must. He is a Jedi, so he tries to put aside his feeling. Several places do we see that he can't manage to do that with Anakin. Anakin meant as much to OBW as OBW meant to Anakin. They were both emotional. Anakin is not unbeatable by the time of ROTS. He just thinks so himself. OBW bested him, simple as that. Vader recognizes this by the time of ANH. Have you guys seen that film? he explicitly says that OBW was the master then, the better one, but NOW he IS the master... arrogant prick Mace wouldn't even be emotional in his fight with Anakin. In the Mustafar duel, Anakin wouldn't last very long I think. Longer in another duel. Give him one more year unsuited and he would be a serious threat to Windu - but not sooner.