Stan's an interesting case, in that they brought him in seemingly with one story in mind (talented, but assholish and of course chauvinistic, with a lot of bluster), but that only worked for so long, especially given that he came in after the boys' club years of Sterling Cooper. So he lost the bluster and was morphed into someone who respected Peggy professionally and personally and had mature relationships with his colleagues.
Harry, on the other hand, started out decent, both professionally and personally, but just curdled as a person during the SCDP years. It’s true that he brought plenty to the firm, but he grew ever more grasping and became an asshole to his colleagues and irredeemably piggish personally.
And speaking of flawed characters, I think Pete was one of the most consistently drawn and complex, start to finish. He could be entitled, jealous, resentful, and manipulative, with a chip on his shoulder, and also competent, innovative, thoughtful, and kind, all of which hewed to the backstory established early on and was reflected in and built on his relationships throughout the show.