Shamans, the only class I leveled to 70 solo, as well as one to 68 and 4 as a box to 70.

I have a hunter at 60, my first character, took me 30 days played time to reach 60. :P (I slacked) and that was awesome too, but it was more because the first time around was just awesome. In terms of classes, I prefered shaman the most. Able to heal, dps, even tank while leveling! If you're the same level of the mobs in the instances anyhow. And doing quite well on the dps part too. :O Enhancement is wicked. XD