Easiest World of Warcraft Class to Level
Hunter (Beast Master spec ones especially) is the most common choice for easy and fast levelling for a multitude of reasons :
- Low dependence of gear to do damage : even with crappy gear you will be able to do decent damage
- Pet class : a pet able to really tank (hold the monsters on him while you're killing them)
- Ranged dps : you don't have to be teeth to teeth with the enemy
- Multiple escape mechanism : Hunters can disengage (jump back), feign death
- Built-in regen mechanism : You're able to regen while doing still some dps, and you're pet is able to DPS as normal while you auto regen
Death Knights... you have 55 fewer levels to grind. ;Þ
I have leveled every class other than death knights through at least level 50 ... and a couple up past 70. I haven't leveled anything up to 80.
I'm interpreting your question to mean "easiest" as in "requiring the least skill and/or attention to solo through to at least level 40."
In my experience the easiest to level has been my paladin. These have good armor, reasonably good combat, and can heal themselves. So you just wander around pulling with exorcism, and then just bludgeoning the mobs with a good mace or sword and occasionally healing yourself until they fall over. When you do a bad pull or get too many ganging up on you then run, use lay on hands when you're near death and keep running. (Yeah, paladins don't have any good crowd control). I've leveled my pally to 61 while while barely paying attention (watching TV, hanging out on StackOverflow, chatting); you really don't need to pay much attention. (Usually I start a fight, go auto attack and let the fight grind down on its own --- with just the occasional self heal).
Druid is the most versatile class. However, you have to pay a bit more attention when leveling and soloing them. Basically you have to watch your mana and switch to cat or bear whenever that gets low (but before your health is too low for that to be sustainable for the rest of the current fight). By regularly shifting from feral to caster you can continuously grind through quests and rarely need to stop for drinking (mana regens while you're in feral forms).
The most difficult classes to play well are rogue and warrior.
To play a rogue you have to employ a high degree of situational awareness ... always using the correct skills (which depend on whether your target is facing you are you're behind it, whether your stealthed, whether they are stunned or sapped, which of your skills is on cool down, whether they're casting (and you need to kick them), how many combo points you've worked up on that target, and how much energy you have. You can't just casually work you rogue up while chatting away in other windows.
However the rogue's advantages are stealth and vanish ... and crowd control using sap. I good rogue can keep a couple, as many as three targets sapped while concentrating on one target (and that's even as low as the mid-20s). A halfway skilled rogue can defeat any other class in a duel ... and most of them are toast even with a five level handicap on the rogue's side.
Warriors are almost as complicated, having to use a mixture of different skills to be effective. You do have good armor ... but the lack of self-healing (just potions and bandages) and the lack of good crowd control makes things more difficult.
Warlocks and hunters are relatively easy to solo. The minions/pets give you a disposable tank and 'locks "fear" can be amazing for crowd control (though it can bring adds). The hunter's feign death is a "get out jail free" card for avoiding those pesky corpse walks. (However, when playing in a party I had it when my hunter survives a wipe with feign death and I just sit around useless until someone else hikes back to res. I'm planning on changing my lvl 72 hunter to engineering and getting goblin jumper cables to at least have some chance of being useful in that situation).
Hunters tend to be in very low demand for groups. They just do DPS and don't really contribute to the party for much of anything else. 'locks are popular for their soul stones (wipe prevention) and their summoning, as well as their extremely good DPS and low maintenance. (You should never have to slow down while your 'lock drinks --- if your 'lock is ever low enough on mana to care then he's incompetent (or he's destruction spec'd which might be the same thing for party/raiding).
You'd think the shaman would be about as easy as the paladin to level. However, their weaker armor really hurts with that. I find that managing totems requires more attention then the paladin auras and blessings. The self-resurrection ankh (re-incarnation) is a popular wipe mitigation feature provided by shamans.
Mages are probably the hardest to solo level. No armor, relatively weak crowd control (just polymorphing), easy to run low on mana, no self-healing, no pets nor minions. Mages should team up with anything that can heal them. Their ability to conjure food and water can be handy as well as their DPS.
I've just recently been working a priest and it's been relatively easy but not nearly as easy as the paladin. You have the healing, but you don't get the armor nor the melee; I'm told you want to solo as shadow spec; but I mostly not intending this to be for solo-ing so I'm sticking with holy/healer. Of course I now have quite a bit of experience with all the low level quests for the alliance races.
In the long run I think the paladin is going to be the one I push up to 80 first. I like the flexibility of being healer and tank (and I am dual-spec'd for it). My wife's level 80 druid is reasonably popular with other groups for her healing and her ability to off-tank or shift to cat form for rogue-like melee DPS (and stealthiness). The ability to shadow meld out of combat (elves) can also be a handy wipe mitigation strategy ... and druids have the only in-combat resurrection ability (rebirth). So I'll probably focus on my druid on the other realm that I play on.
The fastest leveling strategy that I know of is to work quests through about level 16 then just do back-to-back dungeon finder (while only doing the occasional quests while you're in the queue).
Lots of opinion on this one but I would put a paladin up there. Paladins are a defensive focused class that can heal, wear highest class of armor and gets several types of damage shields. They become very self sufficient as they reach higher levels as their retribution abilities rgenerate health and mana. They are also a hybrid class that allows the person to choose any of the three roles of tank, dps or healer which can make finding groups to level with a lot easier.
I recommend a Protection spec Warrior and chain running groups using LFD. I recently levelled a warrior to 80 using the dungeon finder. This was quick, easy, and everyone loved me. :D
Using heirloom shoulders, chest and sword meant I was consistently doing 20-30% more DPS than other classes (even with heirlooms) up until Outland. I felt consistently overpowered.