My friends and I do quite a lot of biking, but most is done on Lamma as we have loads of trails on our doorstep. We have done the ride on Discovery Bay and Mui Wo to Pui O a few times- both of which are very good.
A lot depends on what type of riding you want to do- if you're into downhill then Ma On Shan is the mecca. If you're into cross country then you have more choice with lamma, the rides I mentioned and various others. If you don't have your own bike then the Mui Wo ride is best because the bike shop there rents decent bikes. The bike shop on Lamma rents useable bikes but nothing you would want to depend on!
There is a good website
here which has people organising rides and giving advice. There is also an English guy who works casually (with a fag in his gob!) at
Flying Ball who knows of an English guy who organises rides for fun. If the English guy isn't there ask Philip, one of the staff.
Otherwise you can drop me a pm if you fancy coming across to Lamma- I usually go on Sat or Sun morning, depending on sailing!