Nin/Dark. Ninjas really benefit from a secondary that helps keep them alive; and /Dark does that. /Poison is better at nerfing single targets into jelly, but won't do much to keep your flimsy Ninjas from getting mobbed.
Arc #40529 : The Furies of the Earth
I gave up on my Ninja/Poison MM a while back because they're just so flimsy.
Recently, I dug him out and respec'ed him: I turned him into the primary attacker, with the ninja in bodyguard mode. He's become very survivable and kicks ***...and is fun. The ninja only start fighting when I get attacked, so the Alpha doesn't kill them, and by that point the big bad is debuffed and possibly held.
I've really enjoyed the ninja this second time around, and they are really very effective in this role.
Nin/Dark without a doubt.
Ninjas are very squishy, and Dark has all the tools you need to turn them into tiny little ADD death machines.
Thoughts? I team, like a mixed team, and don't PVP.