Private Parts was the first film I ever saw in the theater where it was just me and the person who came with me. I took my then-girlfriend to a weekday afternoon broadcast of the movie (I'm forgetting why I wasn't working that day), and there were zero point zero other people in the theater.
The film "21" (2008) was the only movie where I was absolutely alone in the theater. I went myself to go see it on a weekday evening, and there was nobody else there. That was even stranger. I felt like I was in some weird dream where the world around me had no people. Once I saw how shitty "21" was, I understood why nobody else was there. Actually, to be fair, the first half of "21" was decent, but the second half was absolutely awful.