Mala, Geiom, Forensix, Jack Sparrow, Ramadanman, Burial, Kode 9, Loefah, Slaughter Mob, El-B and the Ghost team, Benny Ill and the Horsepower crew.
Special mentions for Parson - good variety of styles and far more experimental than most dubstep producers, really pushing the boundaries; Threnody for some quality engineering and dark soundscape business (these would make amazing soundtrack material - really evocative);and Metalboxproducts for that wayward lurchstep and injecting some humour into an often po-faced genre
I've probably forgotten a few so I'll add them as I remember them. To be honest though, there's a whole load of producers that do a few good tunes then a few ok-ish ones so I'd rather big up the most consistent studioheads.
And really we're talking about the best composers not producers here aren't we - good production in itself isn't something that makes me buy records