Well, today was actually my second most, by raw wordcount: 8400 words first draft, writing from about 16.30 in the afternoon to about 1.30 in the morning, with I'd guess somewhere under an hour's break for dinner. (This may or may not seem less good contexted with hardware problems that mean I essentially have not been able to write for three weeks).
My all-time record is 10,500 words, June 20/21 of 2004, Saturday lunchtime to 9.30, going out to see a show that didn't happen after waiting around for it for an hour, getting home furious just after midnight, and writing for three more hours. (I count that as a day. YMMV.)
So what was your most productive individual day ?