Of recent years? It was Nixon but he's been rightfully rehabilitated. Either Ford or Carter, probably Carter due to the foundation work he did with regards to US economy.
Nixon did do quite a few positive things (or at least, refused to stand in the way of positive ideas of others) but the damage to the country and the cynicism of government and authority that was created by Watergate was far, far more damaging than his positive accomplishments.
He was a downright repulsive and repugnant individual. I wouldn't trust him any farther than I could throw six cars.
