Ronald Reagan. Not only was he the best of the last 40 years, but, IMHO, the greatest in the past 221 years (when President George Washington took office). Ronald Reagan was a great leader because he defined the conservative movement, which has sadly been taken to the extreme. He was in the White House when I was born, so I'm a bit biased. He is also credited with helping to dissolve the USSR and put an end to the Cold War. Even though the Cold War didn't officially end until President George H. W. Bush's term in 1991, Ronald Reagan is largely credited with ending it and even that early in his administration. He, Margaret Thatcher, and Mikhail Gorbachev were the instrumental leaders in bringing down the Soviet Union. The relations with Russia still have never been grandioso in the 20 years since the dissolution of USSR.
It is true that Reagan wasn't the greatest at handling the economy and did make some boneheaded fiscal decisions that led to the Crash of 1987, but socially and internationally, his policies were pretty darn good. The only president that comes close to Reagan is Clinton whose star started rising during the Reagan administration.
Do you think illegally arming the Iranians so you can hide the funds and illegally fund the Contra Rebels, whose funding was made illegal in a unanimous vote because they were raping and killing women and children, signs of the greatest President in History??