Easily Truman.
Eisenhower is somewhat overrated in my view. Especially giving green light for Operation Ajax and his relatively weak response to the civil rights movement. I think Truman was a much better president and rightfully is ranked near the top.
From everything I have read, I have never seen an overly convincing argument that Eisenhower actually was "weak" on civil rights ... I mean, sure, he is a naturally conservative and pragmatic guy and he didn't support things he saw as un-Constitutional, but in instances where he DID feel he had that authority (such as sending troops to intervene in Little Rock), he did so.