Gary Sinise, one of the finest actors in the biz, IMHO.

Donald O'Connor. Immensely talented dancer, was allowed to show his true talent in Singing In The Rain, but was then casted in all those Clarence the talking mule pictures. The studio system at work, I guess.

leonardo Di Caprio. Not underused, but I think wrongly used in the past several years. He is a great actor (see him in "What's Eating Gilbert Grape" and "This Boy's life", for examples), but the romantic lead roles he's been doing since Titanic somewhat stifle, by the nature of those roles, his versatile acting talent .

Janet Margolin
Elvis Presley
Stuart Whitman
Robert Mitchum (not under utilised, but probably unappreciated for the being the great actor he was).
Peter Donat
The list goes on.