You almost have to give the reason why you think someone is underrated if you are going to put them in this thread.
Shailene Woodley for me. She is awesome in everything she does, yet despite being a box office star and having proven she can carry some dodgy scripts, she is still not quite A list. She has been nominated for 2 Golden Globes and an Emmy, so you could argue whether she is underrated, but she still feels like she is one dud from being this generation's Molly Ringwald. I think part of it is that she is not classically beautiful.
Natalie Portman - She is only remotely underrated because she has been awful so often for an A list actress that some people don't know what to make of her. She follows up an Oscar WIN in Black Swan with hot garbage like No Strings Attached. What the is going through her mind when she reads scripts?
Couple of tiers down:
Mark Strong - A classic "that guy" actor that can do everything from a Guy Ritchie shoot-em-up to a blockbuster to playing a closeted spy.
Natalie Dormer - smoking hot and very interesting. Not sure why she hasn't gotten a major leading role yet.
Toby Jones - weird looking guy that is an excellent actor
Giancarlo Esposito - Everyone getting a residual on Breaking Bad owes it to Esposito. This would have been a good, not great, show without Gus Fring.