Well it depends what you want to do as each are really strong at the right brackets.
Warlocks are ok in 2s and do really well in 3s and 5s. Warlocks with Shammies can do really well in 2s but they are kinda alright in PVE right now. They are on the opposite end of the DPS ladder.
Druids are pretty good especially Ferals. Balance is decent with the right comp and are pretty insane in PVE with their high damage. Resto is again amazing in PVE, horrible in PVP. Feral druids are pretty amazing in PVP and kinda alright in PVE right now.
If you plan on a Priest you want to go Disc either way. Disc Priests are really strong in Arena in any brackets and are pretty decent in PVE as well. Shadow does alright in PVP and do pretty well in PVE.
Priests are the best are all around for PVE and PVP as Shadow and Disc. Shadow is close to the best DPS spec right now outside of legendary rogues and fire mages while Discipline is a very strong PVP healer.
Moonkins and Ferals actually have pretty weak PVE dmg especially in single target where they are considered close to the worst. In AOE Moonkins are much more viable and a better option yet not as good as Fire Mage/Shadow. Moonkins are very coveted in RBGs for their dotting ability however while Ferals are dominant in arena.
Warlocks (typically Demo) are generally solid in PVE in single target and aoe encounters. In PVP they are also pretty good to use in both RBG and arena
I have the most fun playing my warlock and that is going to continue into Mists as well.
As a warlock in duels It really dep0ends on what class you are going up against as some times we will just get steam rolled. In BG's you will most likely have the most damage done but not as many killing blows. As others have said, 3s and 5s warlocks are very strong. 2s...not so much.
For PVE right now, all three warlock specs are completely viable. In DS there is a best spec for each fight, but with a few exceptions the other two specs are not far behind.\
I've recently started gearing my resto druid and having a killer time with him. I find the boomkin dps rotation boring and so I have started gearing my agi set so I will have a fun offspec.
I have tried to play a priest multiple times but get bored before i get to far into it.