I think most of the "M25" schools are pretty good. My thoughts (based on the occasional visit as a PPL examiner) about some of those mentioned so far:
Denham - TPC. Good reputation, expanding due to the demise of CABAIR. Not overly complex althout the proximity of Heathrow/Northolt can intimidate. Cafe OK, tarmac runway useful in the winter. C152s are well maintained.
Stapleford - commercial backbone. Not sure about the personal-service aspect of PPL training. The times I have been there the place seems to be rather 'rushed'.
White Waltham - good club atmosphere, almost too much. Variety of aircraft. Grass runways, some of which have interesting bumps that can be rather mischievous.
Wycombe/Booker - Tarmac runway and ATC (can put some off). 152s and Warriors. Warriors (ex-BA flying club) are very smart for training aircraft. Seem to be quietly professional.
As someone has already said, Darwinian theory has played its part in the past few years. As far as quality of training is concerned, I would say that all provide a good quality test candidate. Best bet is to go to a few and get the feel.