When Mackenzie started out, a lot of clubs would hire him to provide a consulting report, and then it was up to them whether to do anything with it or not; often, in his early days, he wasn't involved with whatever they implemented. It's a good bet most of his "worst" work came from that period, or else from his days of hectic travel where one of his associates was responsible for everything afterward.
My thought is that most architects their best work will come when they had the best site and the most supportive clients, and their worst will come when they had to deal with too much red tape, difficult clients, or other distractions [which may not necessarily have anything to do with the site]. Generally, though, it's not like we are brilliant one year and dumb-asses another.