I love Dark Pact for leveling (though you cant get it till level 40 I believe).
Put your first 5 points in Improved Corruption. That will make it insta cast. Between your curse, improved corruption, and siphon life (when you get it), you'll have 3 insta casts that you can get off before the battle even really starts. Very handy in PVE and PVP.
My favorite build for leveling up was a drain tank type.
I would often cast immolate first, followed by corruption, curse of agony, siphon life. Now I'd have 4 dots on a mob. Then I'd send in the succubus for some added DPS. Odds are, the succubus wont have agro, so the mob will come gunning for you. Not a problem though, just use drain life continuely until the mob falls. You will lose very little health that way, and if you are low on mana.. just use dark pact and steal it from the succubus (remember, shes not using it, only her whip). Throw in first aid and you have almost no downtime.
