I've been using more or less this exact set up for pretty much my character's entire career, which has been and will be exclusively PvE, by the way:
Life Siphon, Vampiric Gaze, Dark Pact, Unholy Feast, (Bone Minions/Blood Ritual/Cap Sig/whatever), Blood Renewal, Well of Blood, (Res Sig/Resurrect)
I pumped Blood as high as it would go, with around 5 Soul Reaping for good measure, and the rest dumped into Death to play with. Since I got it for a steal, I have Superior Death as well as Superior Blood.
I throw Siphon on a mob or two, kick up Blood Renewal to make sure I have plenty of regen, and just start spamming Dark Pact like it's going out of style. Vampiric Gaze and Unholy Feast are there if I find myself low on health, and Well of Blood for when I find my party could use it. Life Siphon cost 10 energy each and provide 25 seconds of 3 degen, Dark Pact costs 5 energy and deals 51 shadow damage, and Blood Renewal is also a scant 5 energy. All in all, it's very energy efficient, and my damage is never resisted. The only draw back is that despite my huge regen, I can still find myself a bit low on hits at critical moments. A good warrior mob can chew through me pretty quickly, but I suppose that's the case with any caster.
Anyway, I haven't changed this build in forever, and I also don't really touch any skills from Death or Curses or my subprofession (Monk or Mesmer). I've been thinking about Nova bombing with Minions, but that might actually prove less effective in PvE. I've also thought about trying Earth/Air/Water skills to augment my current set with a few debuffs or maybe even a self protection spell, but nothing really jumped out at me. It could be that this build just can't be improved upon, but I was hoping I could squeeze more effectiveness out of my Necro. Soloing is out the question, and I struggle sometimes doing quests with henches. (I've thought about a 55 build, but I lack Spiteful Spirit and I don't think I have the patience for dealing with interrupts.)
Suggestions? Questions? Comments?
Y



Soul Leech is from Abaddon's Mouth btw