Of course this is going to be a matter of opinion, as Clay had both his admirers and detractors, when he was alive and now with historians. But Clay's greatest legacy and reputation is as a compromiser who negotiated America's way through a nullification crisis, a sectional issue in the Missouri Compromise and 30 years later (!!) in the Compromise of 1850, both of which at least delayed the Civil War.
His dogged determination to bring opposing viewpoints to some kind of middle ground certainly would have served him well as President, both in the early 1800s and in 2011.
He did support the War of 1812 with the British and later opposed the Mexican-American War, rightly believing that to add that much territory without a plan for it becoming slave or free states would lead to more conflict between the North and South.
His opponents at the time thought he was too much the politician. They believed he used the system and maneuvered his way through Congress to get what he wanted, or to support his party's candidates. That has never been uncommon though.