Objectively, I think the best final case in the series is Bridge to the Turnabout. But if I had to pick my personal favorite, I would pick Turnabout Goodbyes. By the time Turnabout Goodbyes happens in the Ace Attorney series, Phoenix Wright has only accepted and won three cases, which is extraordinary for a rookie but still a significantly shorter career than most successful people. Enter Manfred von Karma, who not only has won every single case in his career, but has done so for 40 YEARS. Most people would be lucky to sustain the same job for that long. For one to not flub out for that entire time, except for one small penalty, showcases true mastery of von Karma's craft. At this point, when Phoenix Wright learns that he has to go up against Manfred von Karma for his fourth trial, he still takes up the case despite how seemingly impossible it looks to win.
With Manfred von Karma tampering with Yanni Yogi's testimony, Yanni makes it seem like there is nothing more to the murder, and the case almost ends with a verdict. With Larry Butz storming in, being one of the only people to have never consulted with von Karma before a trial started, it threw a wrench in his plans. From there, the case takes many twists and turns. The most infamous is Polly the Parrot taking the stand. Other noteworthy parts include von Karma tazing Phoenix and Maya to get the evidence he wants, the revelation of the case's connection to the DL-6 incident, and the metal detector being shoved onto von Karma's shoulder. The whole case seems like a desperate struggle, but thanks to Phoenix's perseverance and a bit of luck, he was not only able to find out that Yanni Yogi murdered Robert Hammond, but also discovered that Manfred von Karma killed Gregory Edgeworth in the process.
Turnabout Goodbyes is my favorite case for many reasons, but one of the reasons heavily relates to why I go to Smash tournaments. The whole case is equivalent to Phoenix Wright flying in to EVO for Smash Ultimate singles, but is bottom seed in his pool and has to fight top seed, Manfred von Karma, in winners round one. Unbelievably, Phoenix was somehow able to upset Manfred von Karma despite having little experience playing the game, compared to Manfred playing Smash competitively since N64 released. The thrill of getting an upset that large feels exhilarating no matter what, and some people may never let that go. If Phoenix Wright ever joins Smash Ultimate, I hope he's a lower tier to represent the struggle of defending your clients when no one else believes in you. Because, in my opinion, Turnabout Goodbyes teaches players that no matter how masterful you think you are, there's always someone that can upset you at a moments notice. That is what a turnabout is truly about; the miracle can happen.