Dario Franchitti, definitely. Probably Gil de Ferran as well.
Maybe Castroneves or Tony Kanaan, he did a test for Honda once and did quite well.
Rick Mears, A.J. Foyt, and an outside pick: Ken Miles, the man responsible for much of the Ford GT40′S success at Le Mans, in his early days he would have been a rocket in F1, and during a golden era for the sport as well.