I voted for paladin for the easiest/fun factors. I found that there was little a prot paladin couldn't do by him/herself up to 60, including some instances. Even back in vanilla WOW (before BC was released), my guild used to make a huge deal out of any time a paladin died to PVE ("paladins don't die, noob"). Was a lot of fun to grind xps outdoors by pulling 5-7 equal level mobs on you all at once and just AoE them down (shield spike, consecration, retribution aura). At times, it was faster than questing because you didn't have to spend most of your time just riding around, but you need to find good concentrations of mobs.

Hunter was a ton of fun too, but it just took a lot more work if you took on a lot at once. I didn't like having to waste so much bag space on ammo, but they've made that issue pretty much non-existent now (bigger stacks, no need for specialized ammo bags). I'd have to rank them somewhere a little under paladins, just because they are not quite as robust in tough situations. Warlocks are pretty much on the same level as hunters for me.

Enhancement shaman was fun as hell after 40 (mail + windfury, ftw), but only so-so before 40.