Jack Webb as Commodore Wesley, Webb had such a wonderful speaking voice.
Doug McClure as Ben Finney, I think he would have done a better job of being paranoid and personally tortured.
Chuck Conners as Lazaus (although I have a much higher opinion of Robert Brown's performance than most fans).
Clint Eastwood as Captain John Christopher, Roger Perry was okay in the role, but Eastwood could have brought a sharpness to the man who wanted to report and get home.
And Robert Mitchum in any role he wanted, in fact I've thought for years he would have made a great Kirk.