Depends. For tanks and healers, queuing up for dungeons leads to faster xp. DPS tend to do better with PvP or PotD (and those also don't require gear upgrades). Also don't forget to do your job quests, and past 50 you should be doing hunt logs.

Also, may be useful to try the squadron missions at lower levels for DPS. Standard dungeons with instant queue, albeit with sometimes dumb NPCs.