While i abhor Bush Jr, I would put Reagan as #1. He paved the way for Bush Jr. And Biden. IMO Bush Sr is tied for the #3 spot simply for pardoning members of the Iran-Contra affair.
Nixon is one of the worst presidents we've ever had though, and I would put him higher than any of the others. The "war on drugs" caused so many rippling horrors domestically. And those horrors don't compare to what he did abroad.