When picking a class you should consider lvling, endgame, pvp, utility, gold making and how fun it is to play.
From the classes you've listed lock is the best one.
Shines whille lvling, brings a lot to the table when it comes to endgame raids (by a lot I mean portals, healthstones and improved imp) and does decent dps, You can also farm DM as a lock, so you won't have any issues with gold and what not.
As far as downsides go... you will get tunnelled by rogues in pvp and the endgame pve will be you, spamming shadowbolt.
Overall locks are pretty decent.