RFK (in 1964)
Calvin Coolidge (in 1928 and possibly 1932)
Eisenhower (in 1948)
Any Democrats who would've had a better shot at nomination than Woodrow Wilson or Franklin Roosevelt in 1912 or 1932
Any Republican who would've had a better shot at nomination than McKinley in 1896
Any Republican who would've had a better shot at the nomination for Vice-President than Roosevelt in 1900
Of course, my theory is that Coolidge knew the Great Depression was going to happen, so he chose not to run so they wouldn't blame him.
Anyway, I echo the Powel statements. He would have done very well against both Clinton and Gore.