Manohla Dargis, from the New York Times, has Synecdoche, New York at the top of her 2008 Top Ten list:

I tend to embrace my inner Caden Cotard, the theater director played by Philip Seymour Hoffman in Charlie Kaufman's “Synecdoche, New York,” a grievously underloved film about life and death and every agonized and beautiful thing in between, including art and the scratch-scratch of those who are trying to leave their marks on the world. Like Caden, I generally don't see the proverbial glass half empty; I tend to see it drained to the last drop, chewed up and swallowed, jagged shard by shard. (Source)

Film Comment has polled its contributors and other critics, to come up with their own Top 20 list. Synecdoche chimes in at #14, between The Class (France) and Hunger (UK). Number 1 isWendy and Lucy.