Alliance
Pick something that has the ability to do well in both PvE, PvP, and ideally has some little unique out of combat features or systems
Rogue is one, Sub is devastating in PvP, last I checked they were plausible in PvP and of course you have the Pickpocketing side of things, picking locks, etc;. Easy to level as well since you can very easily pick and choose your fights
Hunter would also be one, easy to level, can go melee or ranged, pet or no pet, obviously strong in PvP due to ridiculous ranged mobility, collecting rare pets is a minigame in itself
Druids obviously offer the most variety, dunno how feral is doing in PvP right now but it's not doing great in PvE. The other specs however are doing great, bears are still OP, Balance is sought after frequently, and Resto is Resto, everyone wants a healer
Priests are an interesting one because you have two different ways of playing healer (Predictive or reactive), and Shadow Priests, while not exactly amazing in PvE are very, very good in PvP
Guardian Druid is eaaaaasy on the cooldowns, and all the procs you get are on shit you're trying to spam anyway
Come to think of it Balance Druids are very easy on cooldowns as well, only one that comes to mind is Celestial Alignment which is a three minute cooldown and just gives you 30% more damage
So from the looks of things Druids are your best bet, I can't really speak for feral but from what I remember resto Druids are very low cooldown dependent as well. What you will have to do however is watch to make sure your DoTs/HoTs are still up, but if you can remember the time they last you should be okay
They're both fun, but I've seen more requests for Balance Druids on my realm