I'm going with Derek Raymond's factory series. I only recently discovered these books, and they're simply amazing. Very grim stuff, kind of a meditation on death and dying, but the writing just sparkles, really remarkable. Robin Cook/Derek Raymond was clearly a fan of Raymond Chandler--you can tell from his dialogue, as well as his choice of pseudonym, but I agree with some of the blurbs that he surpasses Chandler with these books. The five books making up the factory series:
He Died with His Eyes Open (1976)
The Devil's Home on Leave (1985)
How the Dead Live (1986)
I Was Dora Suarez (1990)
Dead Man Upright (1993)
That's my choice, and am interested in other people's favorite series.