I'd certainly go along with Kevin Costner as ONE of the worst. There are so many. Stewart Grainger from the old days, John Wayne in almost everything except: Rio Bravo, The Shootist, She Wore A Yellow Ribbon, The Quiet Man, Ft. Apache, Red River and Rio Grande. He made lots of 'em.

John Agar and Shirley Temple in Ft. Apache were pretty bad though.

DougR