Charlie Kaufman and Quentin Tarantino are both brilliant and my favourites. I haven't checked out enough of the more 'classic' directors to make a judgement on whether they're the best or not. Scorsese's great. Out of the recent/more mainstream directors, Fincher, Nolan, Aronofsky are consistently good. Paul Thomas Anderson's been doing some interesting stuff lately -- still gotta check out The Master. I despised Magnolia but loved There Will Be Blood.
I have massive respect for Larry Clark for just doing his thing, even when everyone calls him a filthy pedo. Bully (2001) is still one of the most intense and gritty films I've ever seen.