Very tough call. I'm stuck with Gladiator, LOTR, The Departed, and The Hurt Locker. All of those were excellent in their own right.
There Will Be Blood, for establishing a new yardstick for character development. A lot of the time, "character development" is used for a simple change of heart, or a somewhat-interesting backstory that informs the change of heart. There Will Be Blood actually shows Plainview's personality being slowly whittled away until what remains is as hard and cold as a man's possible to be. The singleminded devotion to showing just that makes the film not for everyone - and I can see why No Country For Old Men ended up taking the Best Picture Oscar that year - but it's still a stunningly original and epic masterpiece that indirectly raises the bar for everything since. (It's like P.T. Anderson saw Michael Corleone's evolution through the Godfather films, decided to one-up it, and succeeded.)
