Dusapin's Etudes and Finnissy's History of Photography in Sound.
Both of these are strong contenders for me (I haven't heard the entire History of Photography), but I would have to choose Rzewski's "The Road" over them, which I'm heard enough of to become a total devotee. It's hard to decide what constitutes a work when you include pieces as large as that or the Finnissy work, which are both multiple hours in length and split into several sections.
Outside of piano music, favorite works written during my lifetime would definitely include the following chamber works:
Charles Wuorinen - String Sextet (1989)
Charles Wuorinen - Saxophone Quartet (1992)
Charles Wuorinen - Dante Trilogy (chamber versions), esp. the Mission of Virgil section which is actually for 2 pianos (1995-1996)
Leo Kraft - Cloud Studies for 12 flutes (1991)
Leo Kraft - Omaggio (1992)
Per Norgard - String Quartet no. 6 'Tintinnabulary' (1986)
Hans Kox - Through a Glass, Darkly for alto sax/piano (1989)
Edison Denisov - Concerto for Two Violas, Harpsichord and String Orchestra (1984)
It's obviously hard to choose just one, and this list will likely change, since most of these guys are continually composing new music and I've obviously not heard everything by most of them. Norgard's 7th-10th string quartets are coming out on CD this month and Wuorinen's second piano quintet (among other new things) will be out by next year. I like a lot of Carter works too, but can't put my finger on any of their dates.