I'm going with Eternal Sunshine for all the same reasons as this Pickle fellow. But I do feel the need to defend Crash for a moment.
Crash has a great screenplay, very well developed and powerful, and it's theme surrounding race is much broader than people realize. Many of the characters go through such major revelations. The car fire scene in my opinion is simply phenomenal, and the pivotal moment in the film for Matt Dillon's character, who I felt so deserved the Oscar over George Clooney for Syriana (which was boring and pretentious as hell!). It's a film that despite it's common theme (SO many movies deal with race), this one really makes you think hard. Script was great, direction was well-done, and the performances shined. Even Brendan Frazer turned in a good performance. Crash deserved it's Oscar.
But the fact is, Eternal Sunshine is just a step above Crash in every way. Each performance is great, especially Jim Carrey, Kate Winslet and Mark Ruffalo, who was hilarious. The screenplay is fantastic, Charlie Kaufman best behind Being John Malkovich, which can't be bested. Overall, Eternal Sunshine is just flat out the best romance movie of the decade.