The great thing about this question is that its not easy to answer. Which says something really good about 4E in my mind.
So far the most underpowered character I've seen is a primal guardian druid. However, while it came across as underpowered, it was by far the most versatile and probably has the most fun dailies of characters I've seen played.
While I'm sure a lot of people will say Warlock, I've only ever seen a Fey Warlock played, and I would say they are the most overpowered race in 4E (followed closely or on par with a Bow Ranger).
So far the most underpowered character I've seen is a primal guardian druid. However, while it came across as underpowered, it was by far the most versatile and probably has the most fun dailies of characters I've seen played.
While I'm sure a lot of people will say Warlock, I've only ever seen a Fey Warlock played, and I would say they are the most overpowered race in 4E (followed closely or on par with a Bow Ranger).