Gary Sinise steals every movie he is in.
Brian Dennehy does too.
I also find myself really drawn to Edward James Olmos. I've always wondered why he wasn't more acclaimed.
Edward Norton pulls off some great characters.
For actresses, I think Whoopi Goldberg never does a bad role. Her character in Ghost stole the movie.
I love almost anything that Diane Lane is in and I think she is very underrated in a lot of ways.
A favorite of mine is Sally Field. At one time she was very acclaimed with some powerful roles as Sybil and Norma Rae, but then a lot of smaller roles that I think she was every bit as good in.