There will be a lot of knee jerk responses to this, but Nixon was a more moderate conservative than Reagan. I think a fairer way to parse this question would be to ask which of these two presidents did more to damage the party, and I think it would most surely be Nixon. While history may grant him some kudos for some of the policies he initiated, his role in Watergate and the surrounding scandal, and his ultimate failure to finish his term shamed not only himself, but the entire Republican Party. I don't think it is too much of a leap to say that the GOP was salivating for their version of a JFK, especially after Nixon, and when Reagan started to gain some momentum, it was an opportunity to overshadow the egregious mistakes made under Nixon with style, if not substance. Reagan was a figurehead that lead to another figurehead, GW. His administration gave momentum and power to the likes of Lee Atwater, Roger Ailes and Karl Rove, not to mention Cheney and Rumsfeld, who collectively seemed to believe that it matters not who is the President. The "conservative agenda" can be implemented by whomever holds the office, regardless of any level of leadership ability.