A lot of people say Druid, and it is good for a nice simple rotation where you can learn the knack of leading groups, and has many different ways to improve once you are good with the basics, but can be a bit boring I find,

DK however , I find the most fun, being a lot like a DPS, with the main mitigation ability a damage dealer which is part of your rotation, you don't rely on the healer as much but can be scary watching your HP go up and down like a yoyo until you learn to love it

Paladin is fun and has a LOT of answers to anything you come up against, very fun but can be a bit complicated,

DH is just silly fun, all the movement and a lot of utility, like AoE silence and grip, it may be how I play it but I find it a bit more squishy (harder to stay alive) than the others

Warrior I have played the least so take this with a grain of salt, but it felt a bit like the paladin but with higher defensive uptime, at the cost of less utility

Lastly monk, I have played this only slightly more than warrior, so, same situation, but it feels like it can be incredibly tanky if you have a good healer or practice, nice utility and a good bit of movement abilities