I've had knight, druid, blacksmith, bard, priest and warrior so my perspective is limited to those classes: my favourite class would be bard.
I've only had one for a few months and it is by far the class I have had the most fun in vs all my other characters combined. I love the guaranteed damage the songs give (makes the horrible hit rate with the scepter is less painful *whine*). I love being able to wipe out numerous little mobs in the same time, I love being able to boost/shield and use magic reflect too. The awesome nature of the class allows me to play hero who rarely dies when saving people from mob (too difficult to be a hero IRL, IG it's easy but you don't get the girl). There are other details which make this class REALLY satisfying to play but I don't want the bard nerfed ._.
Though bard is my favourite class to play, I also my time as a Law knight. It was nice to be big hulking "the tank" of the team. The most enjoyable times on my knight involved dressing up like a virtue guard, drinking an incognito potion and role playing around Brit. I even used this a few times near robbers and they would just RUN, end of the world scattering like flies RUN. If they were looting the corpse of a dead victim and saw me galloping towards them they would scram without even looting the good stuff, it was hilarious

I think I might have remembered someone trying to dispell me thinking I was a summon- this cracked me up a lot XD