My level 50 poisons are:
/dark def
/ice def
/dp def
ice/ corr
grav/ troller
elec/ troller
My Poison/Dark is my #2 main toon and will always be my favorite Poison, but, that being said, I picked a couple of these combos when I still PvP’d, and I would agree with generally prioritizing something with multiple real AoE’s over cones, like /fire, /rad, or /elec. Not that there’s anything wrong with Targeted AoE but something with PBAoE like /rad or /elec is really maximizing that synergy factor. For general play that will be most impressive. But you can also solo AV’s and stuff in which case my Poison/Dark is very good at that 🙂 Water would also be a nice option if you can’t pick between AoE’s of other sets and Dark’s self heal - Water getting both.