I guess it depends on your playstyle.
For PvE I'd say the most rewarding is playing a DPS mage (not pyromancer), it's hands down the most complex DPS rotation out there. Rogues/Warriors are macro heroes and in my opinion that gets quite dull.
If you play a mage in PvE, though, be prepared to be on par or out-perfomed by people who spam 3 macros while you have a rotation that gives you a headache.
If you like healing, then I'd say a cleric, I personally find chloros and bards boring to play.
For PvP I've found that chlorolocks can give the most enjoyment at any gear level (rank 6 now), I haven't gotten my warrior higher than rank 4 and my cleric is only rank 3, so my knowledge is limited. But both seem way more gear-oriented than the former, so you'll get to the 'fun' part of PvP much sooner (as soon as rank 2), BUT that being said, I can imagine a Warrior being more fun when you got rank 6+ and some good gear.
Though, this may all change radically in 1.5 as rogues are getting a huge revamp.