I was a massive fan of the Spyrers in the TTP version. A much smaller gang (children of the spire, sent into the underhive to cut their teeth amongst the denizens to show that they could survive the rigours of Spire-life and its own cut-throat nature), almost always outnumbered, but once they got their combat suits going and gaining a few skills and boosts, they became unbelievably fearsome.
Having an Orrus walking down the opposing gangs heavy, shrugging off heavy weapons, small arms and anything else you could throw at it, resulting in the Orrus beating its target to a bloody pulp with his fists... unbelievably satisfying.
If you can get your hands on it, the old rulebook was put up online for free from the Specialist Games section of GW for download, so the rulebook had plenty of little bits of fluff written in to it that gave a great sense of the dangers of the Underhive and those that populated it, human or otherwise.
Also, Mark Gibbons created some of my favourite art pieces for Necro, including that of the Malcadon: