Well that kinda depends. I always try to play something aesthetically pleasing.

You like knights in armor? Play Tank or Lancer
Asian Flair? Ninja or Monk
Magical types? White Mage, Black Mage, Summoner

If you've played MMO's before you should have an idea of what role you like to play. The learning curve of the game is generally pretty good and gives you time to learn your classes abilities in a fast and good fashion.

That being said I can't help but mention queue times for content. Tanks have the easiest time getting into duties and instances. When i queue tank im usually in a DF in under five minutes. Healers are also generally fast.
DPS however generally have to wait quite a bit longer due to overpopulation of that job role.

So in summary, choose something that "looks cool" to you. If you can only play for an hour or two at a time i would highly recommend a tank or healer class to make maximum use of your time.