The best is the one my band uses at the moment: The Gun Factory in Stoke Newington. Nice clean rooms, good equipment and very friendly, very helpful staff. Good rates and a cool chill-out area too.
The Worst? in my experience it has to be a place called The Triangle in Dalston. The rasta guys that ran it were ok but the rooms were in a terrible state and they all connected and there was nothing in the way of sound-proofing. The equipment was pre-prehistoric! The worst/best thing about it was that one of the rooms was just a set of suspended curtains in the corner of a hall. Fuggin hilarious!
It was only £4 an hour but even if it had been free you'd still have felt like you were being short changed. I wonder if its still there?