Definite FF. He'd've been one of the greats if it weren't for the Conservative Coalition.
Truman had a Democratic Congress from 1945-1947 (an a liberal one at that) and from 1949 to 1953, but didn't really do much of anything with it, aside from purge communists from public life and go on the offensive against the Soviet Union. He had plenty of opportunities to govern in the interests of his base, but ran from them more often than not.
You realize nearly all of his pretty left-wing Fair Deal agenda (including universal healthcare) got voted down by those "liberal" congresses.