As the title says really.
For me, the most underrated is Edward Norton. I think he is absolutely amazing and really brings the art back into acting, especially with American History X and Fight Club.
Most overrated has to be (I expect predictable negs for this) Will Ferrell. All his films bore me to death, primarily due to the fact that they are not funny. He is overrated and stated as being hilarious. I just find him dull and predictable.