Ronald Reagan would have won the nomination and the Presidency, with Ohio Gov. James Rhodes as his running mate.
Birch Bayh, who announced for President in 1975 would have prevailed as the nominee and would have chosen Lloyd Bentsen, who also announced for President in 1975.
Several considerations for the Democrats here:
1. Jimmy Carter would have attempted a run for President. He had his mind on this during the 1972 campaign. He would have been less competitive against George Wallace in Southern primaries in 1976, although he would have kept going longer than people thought he would. But he would not be chosen for VP because, as in 1972, he probably would not have refrained from making statements against the frontrunner (whom I believe would have been Birch Bayh) that he could not take back.
2. Scoop Jackson would not have gained traction due to the opposition from the antiwar Left of the Democratic Party.
3. Reagan was already planning to run for President in 1976 long before Watergate broke. The Republican lean of the Western states toward the Republicans (and CONSERVATIVE Republicans at that) was well-established.
One of the things to think about is that if there had been no Watergate in 1973-74, there would have been no MASSIVE influx of Democratic officeholders in 1974 and a further bump in 1976. Would Gary Hart, Michael Dukakis, Jerry Brown, or a whole slew of Democratic Reps been elected? Ditto for a number of Democratic Governors. And would more conservative Democrats in the South have switched to the GOP, or be replaced by Republicans after they retired? The success of the Democrats in 1976 was (A) not exactly overwhelming, and (B) dependent on an unnatural coalition. The default position of the Democratic Party back then was to choose candidates like Bayh. They NEEDED the votes of conservative Southern Democrats to win back the White House, but the INFLUENCE of conservative Southern Democrats in the nominating process had been drastically reduced by the rule changes that came about by the McGovern-Fraser Commission.