Couldn't stick with one my entire WoW career (which ended shortly after cataclysm), but i'd say the deathknight provided my most prolonged period of "hey this doesn't suck," and I even got to raid as a tank for a few patches. =x
For pvp, playing mage on RMP (aka "license for wins") was the most amusing although that got boring fast as hell. Fuck arena.
Rogue. Started out, rolled up a rogue and played it for five years straight. Sure I had alts, but they are my love.
Of course they are nooby-easy-street, but that's not why I played them. I truly enjoyed the class. I did have alts and other 70s/80s/85s, but none got the attention the rogue did. They had enough tricks to them and I always felt useful; even with how bad we were around the Tier 7 start I didn't QQ and run off to roll Ret. I still busted my ass, I was happy. I have two 85 Horde rogues, and one 85 Alli. Best progress was with Alli rogue, starting at Karazhan (RIGHT before Wrath) and working through raids up to Firelands in Cata (which was when I let my account run out).
Never got in to the healing scene until goblins showed up, and then I decked out a shaman. Always loved how chain heal worked and looked, even if it wasn't the 'go to spam' anymore. Sadly didn't get anywhere.
My first tanking love was Druids. Rolled one up in Wrath, made it through content to ICC with him. Still love them. Paladins are a close second (but I've already shown how much I love easy mode ).
Every time I try to make a Priest, I get bored really fast.
If I was able to get to at least 10 without saying "Fuck this, I got better shit to do" I might actually like it.
Anyway, I got one of every class except the rogue and the priest. Cannot stand the rogues, they feel like weaker forms of the druid cat form. (Granted, I leveled 1-70/80/85 with cat form, so that's probably why I am bias. :P) I usually play the paladin or the warrior. I kinda like the mage though. Being able to run around while casting is mad awesome.
well, me played every character to 85, so got 10 of them... even deleted my 80 warri to create a worgen one, levled it in 5 days from scratch to 85. gotta say, lvling never was easy as today, get heirlooms and you see urself up the hill.
there are characters that are easily capable to manage almost every quest, even hardcore elite group-quests, like for example beastmaster hunter and demonology warlock while other characters can get stuck on those, like rogues and mages, who just lack the healaspect. i know, rogues have blabla, but its just not that much, if the elite-trashmobs cast you to a bloody pulp.
nontheless, every character can lvl very fast depending on your understanding, which depends on your interest of the class, so take a look on what you really like to play and go for it, the best motivation goes a long way if you play something you like.
i like them all. not always and not in every situation, but variation is my key to fun. even in world of warcraft *cough