Sniper / Gunslinger
Operative / Scoundrel
Marauder / Sentinel
Feel confident in that top 3 order, everything else is pretty even as far as DPS specs go.
Op/Sco has the higher upfront damage but sniper specs have long term consistency and less CD's or positioning requirements.
Definetly not operative if you're talking about sustained DPS on a boss.
bit if were talking about lower HP trash mobs, than yes, operative short term burst works well.