Often, those devalue Clinton's presidency do so on moral grounds as he got himself into quite the moral scandal, didn't he? However, strictly in terms of foreign and domestic policy, strength of leadership, execution of the office of president, impact on the affairs of the world and on history, I have to give my vote to Bill Clinton.
I would say Clinton was the best of the three, though George H. W. Bush is often underrated. Both of these men were very talented in foreign affairs, and had the respect of foreign leaders and delegates. Clinton is hyperintelligent, much more so than history gives him credit for (he just, as former aide George Stephanopolous put it, had horrible personal judgement). I think overall, Clinton understood and handled the economy the best of the three. He also dealt with the downsizing of the military very well after the Cold War, and let's face it, America had large budget surpluses during most of his Presidency.
All three of these Presidents suffer terribly when it comes to the issue of "trust." The first President Bush was the former head of the CIA--an agency which specializes in misinformation and misleading others. He seemed to have learned his lessons well there, and his "Read my lips--no new taxes" proved to be the lie that cost him reelection. The nation's economy rebounded from the Bush years in spectacular fashion during Bill Clinton's two terms, and the U. S. was both feared and respected abroad thanks to his leadership. But Clinton faced impeachment procedings for his shenanigans with a female aide that included perjury and obstruction of justice. The second President Bush rushed into an unnecessary second war in Iraq, deceiving Americans with unfound promises of weapons of mass destruction that has cost thousands of American lives. He took the economy that was so strong under Clinton to its worst state since the Great Depression. Choose your own poison, but I find Clinton the strongest of the three. (BTW, I didn't vote for any of them!)
I think it's difficult to compare Presidents. It's really tough and I am not sure there is an exact way to do so. It comes down to what people value and upon what they place their emphasis. For example, even the most harsh critic of George H.W. Bush must admit that he possessed a large level of public credibility following Gulf War I and that the emphasis that he and his Vice President, Dan Quayle, placed on "family values" became a centerpiece of the Republican party, something that can be seen even today in the social issues wing of the party. At the same time, the most intense criticism of President George W. Bush has to involve the fact that he was dealt one of the most catastrophic events in the attacks of September 11, 2001. There is no manual, little precedent, that can be followed in such a setting. When he stands on that pile of rubble with the rescue worker and yells into the megaphone, "I can hear you!," there is a moment of political leadership that transcends time and puts him into the realm of history. While he would receive his share of criticism, it has to be tempered with such a moment of precise leadership. At the same time, I think that President Clinton might receive much in way of praise as a two term President, but the fact that terrorism has become an issue that has dominated American politics of the modern century and was one that was largely neglected under his watch has to be involved in any assessment of his Presidency. In the end, I am not sure there is an exact answer here, but rather one that is dependent on what one feels and what one believes as representation of Presidential success.
This is, of course, completely a matter of opinion. A Democrat will surely say that Clinton was the best of these three presidents. Most Republicans would probably say that George W. Bush was the best of these presidents because George H. W. Bush was not particularly popular, especially among conservatives.
Democrats would say that Bush was not a good president because of the tax cuts that he pushed through and because of all the things he did with the war in Iraq and the War on Terror. They would argue that Clinton was a good president because of how good the economy was during his time in office. However, this is completely a matter of opinion.