Like many people now I have leveled every class to 80. 6 others I've done it twice.

To sum up in order of ease to level

Healers
Tanks
Mages
Rogues

Healers - have all the advantages (healing, DPS) and few disadvantages. My choice for ultimate easy leveling is ToS. Insane healing and DPS all in one little package. By far the easiest class I've ever leveled.

Tanks - have insane survivability. DT can be a bit complicated mechanics wise but all have insane survivability (someone else mentioned they never died on their guardian and I concur - though it's VERY slow at times due to low DPS)

Mages - have good CC and DPS. I personally didn't find any of the mages the "easiest" because I find them glass cannons. If the mobs get to you then it's just too easy to die.

Rogues - by FAR the most painful, hateful, horrible leveling. They have all the disadvantages and no advantages. No healing, no tankiness and no better damage than the other. I *HATED* leveling every single one of my rouges (and I've recently done a ranger to 35 and it's no better at all). Hateful classes.

As mentioned, given a free choice, I'd go ToS. It's a LOT of fun, it does insane damage in the right hands at 80, healing, AoE, CC it's just the all round package.

Buff