In addition to those mentioned (Lonergan, S Coppola, Peele), I think highly of Game Night, Thirteen Lives, This Is Martin Bonner, and Sun Don't Shine & She Dies Tomorrow. Of those, I'd say Seimetz has the most substantial credits as actors, so I'd probably say She Dies Tomorrow.
Richard Ayoade’s Submarine although I also like Whip It by Drew Barrymore, Everything Is Illuminated by Liev Schreiber, Waitress by Adrienne Shelly, The Town by Ben Affleck, Marie Antoinette by Sofia Coppola, Attack the Block by Joe Cornish, In a World by Lake Bell and Zoolander by Ben Stiller.
MAESTRO. Lest we forget the Cooper started around the Stella/WET HOT AMERICAN SUMMER team and must have refined his improv skills there. What's more this is, no, for real, directed, even if mostly I'm grand gestures and I don't mind the formal conceit. Bring back insane vanity projects!
Feels like this needs some French input. Le Grand Bain by Gilles Lelouch, Ne le dis à personne by Guillaume Canet, Au revoir la haut by Albert Dupontel and of course La haine by Matthieu Kassovitz (although this last one is slightly above the 25 years cut-off)
Combed through my favs and answer is this year's insane Rap World directed by Connor O'Malley. Hyper specific milieu that may not connect with folk outside of that environment, but funniest movie of the year by a mile. Available on YouTube
I'm likely guilty of recency bias but scanning my best of lists, it's probably THE BRUTALIST (although AWAY FROM HER and THE MAGDALENE SISTERS aren't far behind).
The Gerwig and Peele pivots from acting into directing and basically not acting anymore (with few exceptions) has been so successful to them, that I won't even say Lady Bird or Nope, because they're thought of now as directors first. Eastwood also exempt. So...
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The Secret Life of Walter Mitty directed by Ben Stiller
By someone who would self-classify as an actor: INGLOURIOUS BASTERDS
By someone who started as an Actor/Director but doesn’t act anymore: 25TH HOUR
https://nanditadas.com/nandita-das-initiatives/firaaq/
Waitress (2007).