Some of my favorites:

Joan Cusack and Carol Kane, both brilliant comedic actresses.

Helen Mirren, at least she is underused in the US. I know her career is much bigger in Europe.

Raul Julia. He had such incredible range.

Some older ones who come to mind:

Jason Robards and Colleen Dewhurst-I saw them in 'Moon for the Misbegotten' on tv decades ago, and fell in love with both of them. Jason Robards had such a delicious dark side that directors seemed to fear, so they rarely used him well. As a TV and stage actor, he did a ton of Eugene O'Neil, who nobody seems to do anymore. Loved him in those.

Colleen Dewhurst just plain rarely got work. That starlet system really worked against some of those actresses.

Another one is Geraldine Paige. She was astounding. Anyone ever see
'The Trip to Bountiful'?