George McGovern. It's hard to say what would be different because constitutionally the POTUS shouldn't have unequal power to the other 2 branches, but with McGovern we might have seen a different end to the Viet Nam war, the Watergate controversy would have been about an ex-president rather than a sitting one. His stance on personal liberties was far different than RMN's, and his foreign policies would likely have also been remarkably different (i.e. less interventionist).

Another key is that if McGovern had won, we wouldn't have had Carter in 1976, which might have made Reagan's road to the White House tougher.

The down side is that McGovern's fiscal policies were very liberal, so that would have left lasting changes over the next couple of decades and set precedent that libertarians wouldn't enjoy.