If you're talking in a PVE context, then assassin for sure when you finally get backstab.
Just spam backstab (get a ranger pet for soloing when you're not grouping) and you can easily out damage a marksmen. This build works especially well when you have high crit rate because you get extra damage from bleed when you crit as an assassin.
I've tested multiple builds and by far this is the best PVE dps. Marksmen does great burst damage but when you get nice crit rate (I had about 19% at lv 20), with Assassin's constant backstabs, bleeds, poison proc, and auto melee damage gets pretty ridiculous.
I played a mainly sin build as well and the above poster is correct back stab is nice. I even found it effective in PVP. Open with assasinate and when my target tried to run just chase and back stab till dead.