I think Zach Galifiankakis will do his best performance yet in Birdman because there have been plenty of articles that have noted the amount of effort he put toward the role and if you continue to put forth effort in general if you keep doing something you improve. Since it's obvious he is still putting forth effort, I think he will continue to improve with each new role he plays.
Yes, I believe Zach Galifianakis's performance in Birdman will be one of his best performances to date because he is stepping outside of his comfort zone, out of the comedy world. I think he is a great actor and it will be a great stretch for him to showcase his acting chops.
I've actually been following Zach Galifinakis's career ever since he was on that show Tru Calling, and he's quite amazing. His dramatic acting might actually be stronger than his comedic acting, and given a role in a movie in which he can truly shine (unlike many of his other dramatic movies, which have had sub-par writing) will probably contribute to the possibility of this being his best role to date.
Until now, Zach Galifiankakis has primarily been known as a comedic actor, and an extraordinarily silly one at that. He has starred in innumerable comedies, as well as his hilarious, but bizarre internet series, "Between the Ferns", which has pushed the boundaries of comedy as a genre. With this new work, Zach will prove to the world that he can also do serious, dramatic work, and that he is worthy of more respect than perhaps he gets from fans outside of the comedy world.