I used to go to Cameos, just behind Oxford Street, which was sickkkkkkkkkkkkkk for hip hop, bashment and rnb- prob the best in London for a season. However this was a couple of years ago when I was into clubbing a lot more, and I've heard it has since gone from ghetto-fab to just ghetto. I have no idea what state it is in now, but might be worth a look into just in case it's back in vogue.
Now, more than before, it is more about certain events that are held only on certain nights in various venues, so you'd be in good stead to keep up to date with that, as the same club can have very different crowds and music on any given night. There have been some massive events at Indigo in the 02 stadium, held by Choice.FM DJ's for example.
Anyway, this is a good site to browse: http://www.viewlondon.co.uk/clubs.aspx