Viggo Mortensen isn't really underrated, he doesn't really take mainstream roles and has a billion interests and pursuits outside of film, but he is by far one of my favorite actors and always worth seeing.
Oliver Platt. Has a main cast role in Chicago Med, has been in The West Wing and Modern Family. He was in Dr Dolittle, and nominated for best supporting actor in a Comedy. He's also been in movies 2012, Frost/Nixon and X-Men: First Class.
Ethan Hawke is never viewed as a top tier actor, even though he should be. Tom Cruise doesn't get nearly enough credit for his acting. Jesse Eisenberg, like Tom Cruise, is one of those actors where people say "he's the same in every movie" even though he clearly isn't to anyone who actually watches his movies. Timothy Olyphant should have been an A-List actor. That is all.
Oh Vera Farmiga is up there. First saw her in the Departed, loved her in Up in the Air, wish she was in more. Source Code is solid, and have considered checking out Bates Motel because of her.
They've already been mentioned, but Michael Shannon and Sam Rockwell were the first I thought of when I read the thread title. I will pretty much see anything with them in it.
Came in here to mention Ben Foster and am happy to see many others are on the same page as I ! This guy is just the right role away from being an Oscar winner -Hugh dancy did turned in a killer performance in Hannibal -Tatiana Malaney is an absolute powerhouse and someone needs to let her helm a some films dammit -Keith David. Nuff said
Ben Foster was horrible in Warcraft but that's not entirely his fault. He was great in the Messenger.
Joe Gilgun is very charismatic and has a pretty distinct knack for comedy. He isn't in a lot of things compared to other actors but I'm always happy when he is, and he's probably the best part of the Preacher TV series so far.
Love love love Michael Shannon. He goes from being the biggest asshole in 99 Homes to being a loving father in Midnight Special, and he pulls you in to give a shit about both almost effortlessly.
Yeah, he was really good in that. He was also amazing in Mean Creek, and he had a part in a goofy thriller movie called ATM where he blew the rest of the (three person..) cast out of the water
This feeds a little bit into something I said earlier about movie stars being known more as movie stars than as actual actors, but recent viewings of Some Like it Hot and Gentlemen Prefer Blondes have made Marilyn Monroe click for me. The fact that she was an actress feels like an afterthought with her, she's remembered more as a sex symbol, a beautiful famous woman who sang and was in movies but is remembered for being beautiful and having affairs with famous men, rather than actually being a great actress. Her Gentlemen Prefer Blondes performance is so great, she's playing a ditzy character but it's so hard to get a read on what she's actually thinking or feeling, how much of her is genuine and how much of her is scheming. It's an iconic performance, and I don't know that, as an actress, she's remembered for how great she really was.