Hmm well from the 3 classes u mentioned earlier
Mercenary- Nice rdps and aoe, Also has the option to be a healer. (This is the class ppl complain about spamming tracer missile). They also wear heavy armor.
Sniper- Rdps, more of a single target. (I wouldn't know how complicated ppl fix up this class when they hit 50 since I haven't played this one yet, only played with)
Operative-Stealth mdps, Also has the option for a healer. (Someone else should also tell you more about this class since I havn't played this either )
You said you played a warlock before, I would recommend checking out Sith Sorcerer. Caster type that shoots awesome lightning and can heal.
Lastly since you also said you had a warrior. Um There's the Sith Jugg and Mara. Played both to 50 and I used em as Dps. You can play a Jugg as dps but there are still tanking elements there so maybe not this one? Mara took a little longer to learn how to play effectively. But once you can play them well they are awesome lol.
Uh I know I didn't really give you one definitive class so I'm sorry for that, but the best I can do was give you a quick glance on your choices.
Hope fully this narrows it down a bit for you