I know many will say "Play what you like!" but since you've already known about that, I'm sure, I'll just skip to my (very limited) view on what you asked:
Well, if you're looking for a DPS class, I would say that a Mage is always liked on raids; whether single target DPS or AoE DPS, you can't go wrong. On top of the free water (that you'll be sure to make beforehand...) for others, de-cursing ability and portals to major cities for any random reason, thus saving people from using their hearthstone.
Then Warlock for similar DPS, on top of their summoning ability and Soulstone ability!
I've seen raiding guilds or PUGs asking for Hunters too, but not as much as Mages or Warlocks in general. I think ranged DPS is preferred usually (?). Highest char I have made is 40 Hunter. I'm only going by what I randomly see in World chat on the PvP server at the moment. Mages and Warlocks are always welcome in dungeon groups too, for similar reasons spoken of above.
Anyway, if there's no other hint you can see from World chat (type: /join World on the chat bar when you log in), Trade chat or anything on forums, I'd suggest a good old Mage. I find the Mage a lot of fun to play anyway. Squishy, yes, but still fun!

My main is now a Mage (30 at the moment), simply because I find Mage so fun to play. I was getting sick of the Hunter. I don't know, maybe will level him later after Mage is 60.