I hate giving an honest answer here, because it’s going to come off so film-buff cliché, but… it’s Citizen Kane. Since first seeing it in my early twenties, I’ve watched it dozens of times. Gregg Toland and Orson Welles invented the look of modern cinema in 1941, and I’m continually amazed by it.
Close runner-ups are Blue Velvet, Goodfellas, Fargo, There Will Be Blood, and Pulp Fiction, among others. My top 20 are all good enough to be my favorite, really. They all matter to me in some really formative way.