Monk, for certain, all three specs.
I also plan to have a warlock on the side, a class that is never a bad pick, since guilds need atleast one warlock due to healthstones and the incredible utility gateway provides.
I may also put some effort in keeping this mage of mine up to date, if the expansion will not be terribly grindy.
As for recommendation for DPS, there are a few who have always been good throughout the raid tiers:
Mage - Decent mobility, Timewarp, Iceblock for soaking. Choose this if you also want to do mechanic handling on the side.
Warlock - Good utility, and one spec is always top/near top in damage. Also pretty sturdy on the side, but not as much as mages can be.
Hunter - Generally middle of the pack, but they have some pretty good utility tools that makes them good substitute if you are lacking one of the few essential classes. BM is also one of the easiest ranged spec to get into, double so if you also want to do mechanic handling on the side because you have no hard cast abilities/spells.
Rogue - Never a bad thing to have, but I feel in raids they often only have damage to bring, other than the ability to have access to a soak ability on semi-low cooldown. They really shine in dungeons (and likely in PvP) due to all the CC they have.
DH - Can be a good option if you want a semi-simple, high mobility melee DPS. You will always be needed in some form (tank or DPS) even if you are a mediocre player due to your passive magic damage increase. You can also go for this if you like your melee to be flashy, and if you are too much of an edgelord to go DK.
Other classes have their ups and downs, but there is always a market for mages and warlocks. Warlock is the safest bet out of them all if you do not mind the hard-casting nature of all three specs.