Depends, do you enjoy DPS / Tank or Healing? Thats the big question you need to ask yourself.
If tanking is your thing i'd be careful as most raiding guilds probably have well established tanks so once you hit 60 and want to actually raid as a tank it'll be a hard battle to find a full time guild i think, still though if its what you want to play then play it, don't play a class just because people tell you to etc.
If its DPS, you need to think, Melee or Ranged? Both have pro's and con's and it's really what playstyle you like, Mage is a good choice as they're usually always in demand and do good damage.
I'd advise against rogue unless of course you're super dead set on it as there are tons of them, same goes for warrior dps.
If its Healing i'd advise Priest or maybe Paladin, both are really strong in both PvE and PvP, of course rolling any healer will get you spots anywhere and it all depends on what you like to play.
I personally chose Priest due to them being great in both PvE and PvP and being the strongest healer
