Which president was a better as a Statesman than as a President?

James Buchanan comes to mind. He was Secretary of State under Polk and negotiated a peaceful settlement to the Oregon boundary question. He served as ambassador to Britain and and had a long diplomatic career with some success. As president he could think of nothing to do, even when
states began to secede from the union.
One might also argue that John Quincy Adams was more effective as a statesman than as President.