booster17 writes fanfic in a wide variety of genres, usually Buffy-centric and relatively light in tone. His stories are fun, and can be found Here.
cdybedahl mostly writes slash for BtVS and other genres, including some crossovers, ranging from serious in tone to very funny. Her BtVS / Sapphire and Steel crossover Assignment Sunnydale is VERY memorable for anyone who remembers the series. Her stories are here. Although she isn't a native English speaker her spelling and grammar are probably better than mine, and her plotting and characterisation is excellent.
speakr2customrs has written humour and a few more serious stories, all well plotted and written. I particularly recommend And Glory Shone Around, in which Glory decides to have a Christmas truce, Merry Sport, which defies description, and his Buffybot stories, all here, and the wonderful BtVS / Noggin The Nog crossover Noggin and the Vampire, which is here
There are loads more I haven't mentioned, and if I haven't included you here please don't take it personally - I'm just running out of steam on this, and I think in most cases you've been recced by many other people.