Others have mentioned it, but there is a difference, and often a significant one, between the skill floor and the skill ceiling. Some classes are easy to pick up (I personally believe they all should be), but difficult to actually master (again, I personally believe they all should be).
A Frost mage, Elemental shaman, Frost death knight or Arms warrior is probably the easiest spec to pick up. Out of those four, and considering what I've actually played, I'd say the ceiling for Elemental and Arms is pretty low which would, arguably, make them the easiest. We could argue that Fury warriors and Destruction warlocks are also easy because they are... On the face of it. But when you move into the top-level micromanagement that those specs require, they stop being so simple.
It's different strokes for different folks as well, though. I tend to play ranged classes pretty badly, whereas I'm really good at picking up melee classes; I'm sure many others are the opposite way round.
It depends, hoho.