I would disagree with Denethor. He was a man as Tolkien said became corrupted by politics, but he was quite good at what he did. All the more amazing still remaining to keep sanity (for the most part) while viewing the palantir for 40+, dealing with the death of his wife and son, and his other dying son, as well as seeing his kingdom on the verge of destruction. But, that's just a matter of differing opinions, it's definitely easier to dislike Denethor, then it is to like him.
I would vote Fengel. Just because he wasn't very well liked. He caused strife with his marshal's, his own son hated him so much he went to Gondor and was reluctantly returned back after his father died to take his place. He loved gold, cared nothing for friends. Sounds very stupid to me, and quite foolish.