What class is under played the most?

[Striiker]

Pick a class which you would like to play!

If you are more concerned about grouping, play a class from the "holy trinity" Tank (any of the three), Cleric or Enchanter. When I am on with one of those classes and have to leave and can't find a replacement, it can really hurt a group to the point where it breaks up. :-/ (so I always do my best). Clerics can be replaced by other priest classes though in a pinch.
Regardless of what you choose, there will always be times when it's tough to get into a group. It's best to start a group on your own in these cases but that can be a challenge too if people don't throw on their LFG tags.