If you're looking for dark sci-fi, you can't go wrong with Warhammer 40.000. They're not all winners, but in my experience just about anything by Dan Abnett, Graham McNeill, James Swallow and Aaron Dembski-Bowden can be considered top of the line.
Special recommendations:
Dan Abnett:
- Eisenhorn (Omnibus) (this is what got me hooked on W40k)
- Ravenor (Omnibus) (sequel trilogy to Eisenhorn)
- Brothers of the Snake (very good one-shot)
Aaron Dembski-Bowden:
- Cadian Blood (one-shot)
- Soul Hunter (start of an ongoing trilogy)
Aside from that, the ongoing Horus Heresy series of novels is also great, with a few small exceptions. I'd recommend you start with a one-shot to get into the groove, but if you find W40k to your liking, then this series is pretty much a must.