I haven't played Wrath since it was current so I'm totally out of touch with how the classes were then, as they are totally different in current retail, Outlaw rogue anyone?...
When wrath was current I had a lvl 80 Prot warrior (alliance) lvl 80 Boomkin druid (horde), lvl 78 Combat rogue (horde) and a lvl 32 Shaman (alliance). I still have these classes today on retail.
I've narrowed it down to 2 dps spec: Elemental Shaman or Shadow Priest (never a fan of warlocks, mages and don't get me started on DKs...).
How are they both? I don't care about being top dps, but more about their usefulness in a 80 dungeon or raid.
Shadow priests will always be useful in a raid, also dealing very nice dps and being quite crucial in certain situations like 25HC Halion. They can be a little tough to gear as you share many items with most other casters (shamans do with a couple mail pieces which can ease things up). It might get annoying in a high-end guild as they will likely ask you to switch holy at Valithria, so you'll constantly respec if you want to pvp as well. Elemental shamans aren't very popular unfortunately, they are decent but it's generally preferable to have a resto sham in the group and have a demo lock/balance druid give the other raid buffs. So while technically easier to gear and also easier to play, you might have a tough time getting in raids with a decent guild. I would recommend a priest, you can't really go wrong with them, and in my opinion it's also more satisfying since there is a lot of optimization to figure out in their gameplay. GL HF :)