Roadkillowns of Aerie Peak posed the question on the official forums, what is the easiest class to level? Hunter came up most frequently as an easy class to level followed by Warlocks. They usually have very little downtime. Protection Warriors and Holy Priests were often quoted as the most difficult to level.
Having leveled a Shaman, Paladin, Hunter, Priest, and Druid to level seventy myself I have to say that as a solo character, the Hunter was hands-down the easiest for me. Between my pet carrying agro for me and the glorious Feign Death ability, I was always good to go.