I've loved levelling all 3 of my 70's.

My warlock was my first toon, so I made loads of mistakes levelling (so it was quite hard). After discovering affliction it was easy mode.
Drain tanking 3 - 4 mobs the same level as you, killing them and having close to full health and mana for endless grinding was awesome.
Then of course the coolest looking pet in the game the felguard.

My second 70 was a hunter... Beast Mastery. I would say it's easy mode. I managed to get to 70 without much thought. Took out elites with ease and rarely died. Did all this without really having to develop any of the skills of a hunter (such as kiting). Learnt them afterwards.
My hunter was on hold, but I took him through kara on his second full run, and despite being pretty poorly geared he gave a good solid performance.

Ah, now, my third 70 my shaeman. I enjoyed levelling him. A completely interesting class. Such utility in later levels, and I levelled with a friend to the mid 40's. Two enhancement shaman can cope with just about anything. After that solo levelling and it was pretty good.
I switched to elemental on going to outlands and thought it sucked. Until I started getting gear with spell damage on it.

So long as nothing is hitting me the damage is pretty amazing.

So currently it's the one I enjoy playing (that may be due to freshness).

It was the hardest of the 3 to level on it's own (mana issues until quite late on). Mid 60's onwards seemed to go real quick.

Most other classes I'm lucky to get to 30 on, but will go back and level my priest (30), my mage (36) and rogue (45).

(spot the altaholic)