Everyone's doing it, so I'll jump in.
To be fair, this only really works if you've seen all the movies made in the last decade. I haven't. But I've probably seen more than your average Joe. I don't like a dark, spare film, so take heed. Any films coming out now-ish are not in the mix.
in no particular order...
No Country for Old Men
City of God
Zoolander (whatever you think, I don't know if I have ever laughed harder, not even at The Hangover)
Night Watch (despite choppy editing and confusion--helps to read the book)
Eternal Sunshine of the Spotless Mind
Monster House 3D
Most underrated film of the decade: The Darjeeling Limited
Most awesome contributor to film this decade: Clint Eastwood, specifically for Million Dollar Baby and Mystic River (despite the heinous accents) and, perhaps, Invictus, which looks great.
Other great films: District 9, Wedding Crashers, Wall-E, Memento, Monsters, Inc, Little Miss Sunshine, The 40-Year-Old Virgin, The Cooler, Coraline, Almost Famous, The Emperor's New Groove, The Lord of the Rings trilogy, Spider Man, Dirty Pretty Things, The Triplets of Belleville, Mr. and Mrs. Smith.
There were certainly other great movies, but either my ardor has cooled for them over time (Slumdog and Crash I still love, but I have other movies I prefer to fight for) or the minor flaws have just become too large in my mind (Leo's overacting in The Departed). Other movies I realize I loved only because of an actor's performance, not the film as a whole (re: Helen Mirren, Ryan Gosling, Robert Downey Jr., Jamie Foxx, Johnny Depp, Amy Adams). And Milk would be on here if its structure was a little different.
So take it as you like it.