Kevin McCarthy or Richard Kiley?
From trivia- "Gig Young, Dick Powell, Joseph Cotten and Richard Kiley were all considered for the role of Dr Miles Bennell."
Of those only Richard Kiley could have been as good as Kevin McCarthy imo. I also think John Forsythe would have been good in the role.
The 3 actors have some similarities- all are a little above average height (about 5-11) and athletic. All were and looked highly intelligent.
Richard Kiley had such a great voice. He would have been good in the role. I won't go so far as to say he would have been better though.