Well having read what preceeds, I think I have to disagree with nearly all.
I use the Viks and Sonia as my primary masters, and find both equally good. Obviously some masters give them a harder time than others, but this is true of any master in the game.
I also have Levi, and he can prove difficult to beat, but I would say its a lot harder work to actually achieve a win with him. He requires a very high level of concentration I feel to get him working well
I also agree that Ramos feels like a master it could be hard to win with, he just appears very slow, but then I have only played against him, not with him.
To be honest, I think most of the masters can win, its more down to what strategys you and your opponant get and how your schemes pan out in conjunction with your crews choices. heh a cop out answer....