Wow! A tough one! Could go along with many mentioned on here. I disagree that Scorsese has done no bad films. I think he's done some real dogs (God save me from ever having to watch one minute of Gangs of New York again), and one or two that just don't merit the time given over to them. They could have been edited by half-an-hour or so with very little loss (I'm looking at you, Aviator! I'm also looking at you, Irishman!). And I am a big, big Scorsese fan going back to Alice Doesn't Live Here Any More right through to his latest, Killers of the Flower Moon (again, maybe a bit too long?).
Scorsese is also a terrific educator, you know. He knows his film history backwards, forwards and sideways, and speaks very well about other films, some of them unjustly forgotten. Although his strong suit is American cinema, and Italian cinema (unsurprisingly).
I've not really followed his career through, but I remember being hugely impressed by early Terrence Malick. I suppose I know just about line and shot in Badlands.
Welles is a good shout, but his actual film output doesn't last that long, does it? Everybody knows Citizen Kane, but The Magnificent Ambersons is just about up there with it. Incidentally, for those who want their Shakespeare accessible, and even seductive, Chimes At Midnight well worth discovering, for those who don't know it.
Among those no longer with us: Lean, obviously the British director who achieved true greatness (although not with everything — Ryan's Daughter can look almost schmaltzy in places). But if he'd only done Lawrence, he'd have his place in film history. The film that inspired Spielberg to want to be a movie maker.
Hitchcock, very good in his way — but more than half his career is American. Not that that's a fault.
Wilder — as someone I revered, I was surprised to find late on that he made one or two fairly weak films, too. Obviously, Some Like It Hot is one of the treasures of film making. The Apartment too is a gem. Oh, and there's the little matter of Sunset Boulevard.
Among living directors: very good shout for Paul Thomas Anderson (props to the good Dr). But for me the greatest living director, hands down, is David Lynch. Disturbing, thorny, sometimes downright uncomfortable — but always challenging.
Ridley Scott is a supreme craftsman. So is Spielberg. They're very, very good storytellers. I enjoy their films very much, but I don't think of either of them as great film makers. (I make an exception for Blade Runner — if all films were destroyed by aliens somehow, that would be one of the ten or twenty that I would save for other civilisations to know what the human imagination is capable of and — more astonishingly — can put on celluloid).