For me, it's Blade Runner 2049 easily. I saw it in theatres on a whim and I was the only person in the theatre and it was an amazing experience. Especially since it was in IMAX 3D, I sat in the middle of the room and got to properly enjoy it. Not to mention the fact that the film itself is a masterpiece in my opinion, from the acting to the VFX, the sound design, it was near perfect in my opinion and I was disappointed that it sucked at the box office. Movies like that are once in a lifetime and I wish it had the commercial success to back it up.
Anyway, tell me your favorite underrated gems. I need to have a marathon and watch them all