I don't think Percy or Ford would run at all because they are both up for reelection to Congress (Senate and House respectively). In fact Ford wanted to be Speaker, not President, and took the VP position once he realized that he would never become Speaker and VP was the next best thing.
That said, Rockefeller would be tainted by his losses in '64 and '68 but a comeback is not impossible. Reagan is more likely than any other option to be the nominee, however he has no chance of getting elected in 1972. Provided that whoever is elected in 1968 ends the war and the economy is still good, they would be reelected in 1972. And as I explained on a similar thread I created about 1976, Reagan was too right wing even for the GOP during this period. It took Watergate, Carter, and a split moderate establishment in 1980 for Reagan to win the nomination and then the Presidency. The country is still governed under the liberal consensus in 1972, so his chances are slim at best.