i think there's different types of "dark", for one thing. first thing that comes to my mind for dark is PCP / The Mover. Lots of very dark, reverb-heavy techno. But that's a lot more gloomy and melancholic than a lot 93 darkcore which is more sinister-dark.
This is a nice tune that sort of fits somewhere between - it's 93-style, although it's actually an Australian tune from 2003:
Probably the darkest rave-related thing I've heard is maybe the Venetian Snares - Doll Doll Doll album. In particular, "All the Children Are Dead" and "Befriend a Childkiller". (love the intro...very eerie!)
off the top of my head the 2 darkest UK breakbeat tracks that spring to mind are both by Sound Corp - Regen-Time and the Toll,
If we're talking hardcore techno then there are loads of darker tracks, lots of PCP stuff in particular the Frontal Sickness series by The Mover, anything on Cold Rush Records. Also Brainkiller by the Mackenzie and Aftermath by DJ Repete which were the first doomcore tracks produced.