Probably not a difficult question. Eternal Sunshine came in at #43, between Donnie Darko and Predator. That's a trio you don't see every day. Kind of an odd Top 5, too: Bladerunner, Alien and The Empire Strikes Back are the top 3, The Matrix is #4 and 2001 rounds it out.

Memory-tampering is a genre staple often reserved for amnesiac thrillers and mind-bending actioners. Not so with Eternal Sunshine, director Michel Gondry and writer Charlie Kaufman instead using it to explore the nature of the human condition – in particular heartbreak. What happens when love goes sour? And what if you could erase the memories — both bad and good — from your mind? Would you go through with that? After learning his ex, Clementine (Kate Winslet) already has, Jim Carrey's Joel decides he will too. But as he explores what made them meld together and then fall apart, he starts to realise that he still has feelings for her. If its tech is fictional, the emotions in Eternal Sunshine are completely real. (Source)