Interesting question, as I'm a *HUGE* genre fanatic and collector (I have well over 700 DVDs; not the biggest collection in the world, but all mostly Giallos, old school Slashers, Zombie epics, Cannibal tales, Cult films, stand-up comedy which I feel is an art fourm, ect).
While I believe it's all a matter of taste, really, as to WHO the official Master of Horror is, I have three personal favorites.
And those personal faves are:
John Carpenter
George Romero (has always remained in Pittsburgh, and now Toronto, and refuses to "go out to Hollywood to get paid to fart")
Dario Argento
While I feel that they're better days are all well behind them, I still feel that all three are living legends, and the line forms well behind them in my humble opinion.