Well I don't knw about the others but B\X I can do. I have spent hours searching the web in the last couple of weeks for great B\X fics for my new sit Can I Have You?

For long Epics my favorite is Lost Destiny Series. It's long and slightly AU but the characters rally hold to theire BTVS counter parts. Excellent on all accounts.

For tons of smutty fun I also recommend The Cure Series. Starts off as PWP but ends up with a good emotional plot.

Hope that helped!