Either Three Flavours Cornetto (Edgar Wright's trilogy of English genre comedies Shaun of the Dead, Hot Fuzz, and The World's End) or the trilogy from which it took its name, Kieslowski's Three Colors (consisting of his final three films, Three Colors: Blue, Three Colors: White, and Three Colors: Red.)
Cornetto is where my heart tends to lie because at the end of the day, I'm a fan of genre film first and foremost. Its rhythms and its tropes are like a well tuned drum beating constantly in the back of my mind while I'm watching a genre film; it's very pleasant to see a director playing the genre conventions and using them to create something really unique. Using zombies as a catalyst for breaking a slowly aging man out of a suburban rut is inspired. Using action tropes and a vague murder mystery plot to satirize the insanity of suburban groupthink still blows my mind. And using an alien invasion to mask a deeply affecting portrait of alcoholism and self-doubt is still one of the most insidious ways a movie has ever given me an existential crisis.
Three Colors is up there almost entirely because Kieslowski was my first real introduction to art film. It was the first trilogy that really let me see the strength of the format and the possibilities it offered beyond a single story being told in three parts (which is what most trilogies are; Star Wars, bless it, barely feels like a trilogy because Empire and Jedi are ever so slightly out of step with the original film, a byproduct of being afterthoughts made after the wild success of the first rather than conceived as part of the initial vision). Seeing how each film fit into Kieslowski's overall inversion of standard structures, playing jump rope with expectations of comedy, romance, and traditional drama, is one of the things that made me fall in love with art film. After Kieslowski came Tarkovsky and Kiarostami and Vertov and Vinterberg and Park Chan-wook and Wong Kar-Wai and Weerasethakul and all these wonderful wonderful directors that I would never have been exposed to were it not for Kieslowski's brilliant work. He blew the doors wide open for me, and I will always love these films for that.
Honorable mention to Richard Linklater's Before trilogy (Before Sunrise, Before Sunset, Before Midnight) for being deeply affecting portraits of relationships over vast stretches of time. They've arguably shaped how I approach human relationships over the course of my entire life.