As others have said, priority should be to get GLA/PLD to 22 for provoke before really starting to tank. It's beyond me why such an essential tanking skill is given to only one tank class (it would be like raise only being given to one healer, but having the others cross class it), but that's the current situation. If you want to go all the way to 34 that's good, but awareness is by no means vital until endgame or you get Raw Intuition at WAR level 56.
From there, it really is down to personal preference in style. I have all three at 60, was PLD main through 2.x (when I was playing as I took several breaks) then did DRK through the Heavensward storyline, before taking PLD to 60, then switching to WAR for raid group synergy. PLD is very easy to get to grips with, has straightforward cooldowns which are a) always available unless used; and b) don't require resource to reach full potential. The animations look cool and it definitely has a good feel to it (in my opinion), but it is generally a little lackluster and falls behind the others in damage (especially AOE) and enmity generation. However, if you don't plan on doing bleeding edge progression, it's fine.
DRK is a fun class to play, with plenty of action, but to me it felt a bit too much like a micromanagement dps class with PLD cooldowns thrown at it. Of all the tanks, I actually found this to be the most difficult having to juggle both MP and TP along with planning ahead with MP to ensure a CD can be buffed by Dark Arts when needed. It's an interesting class, and I think I'd really enjoy it if given the chance to main, but it takes some getting used to.
WAR is my current main and I find it to be the most refined of the tank classes. All of the abilities sync well with one another, and it doesn't feel like there is much in the way of bloat there. I find with both DRK and PLD their abilities are just a bit bland or too clunky. Carve and Spit for instance - either some oGCD damage or some MP restore, but doesn't really feel as satisfying as hitting Fell Cleave, despite having nearly the same potency when buffed (if it actually did three attacks at that potency, then I'd enjoy it!). Divine Veil should be a great boon to the party, but falls a bit flat when you have to jump through hoops to get it setup and ultimately isn't all that much more effective than just hitting Storm's Path before a raidwide AOE. Chances are the healers will overheal everyone anyway, so you might as well get 10% less damage overall for 20s than a short HP shield.
My suggestion would be to get GLA to 22, possibly unlock PLD and get to 34 (although PLD pre 40 is a bit painful as you have no tank stance so lower enmity generation than the other tanks, despite the DPS classes getting some nice DPS moves in the mid-late 30s). From there I'd probably try and have all tanks somewhere around the 30 mark (you'll want Mercy Stroke from 26 MRD in any case) after which you'll have an idea of which you enjoy.