I can't choose just one...
In no particulate order...
All Star Batman & Robin Vol. 1
Batman #515 to 552 (1995-1998) - the Doug Moench/Kelley Jones run
Batman Illustrated by Neal Adams Vol. 2 & 3 collection of the Denny O'Neil/Neal Adams run
The Dark Knight Returns
Batman Vampire trilogy (Red Rain, Bloodstorm and Crimson Mist)
Arkham Asylum Serious House on Serious Earth
Batman Chronicles Vol. 1, 2, 3 & 4 collection of the original Bill Finger/Bob Kane stories
Gotham After Midnight
Batman Year One
Strange Apparitions collection of the Engelhart/Rogers run, plus Len Wein's Clayface story
Dark Detective
Killing Joke
DK2
Gothic
Batman In the Seventies with There Is No Hope In Crime Alley by Denny O'Neil
One-issue classics and two-parters that are still uncollected into paperbacks:
Detective Comics #451 (1975) - The Batman's Burden! by Denny O'Neil/Ernie Chua
Batman Family #19 (1978) - The Tomb of the White Bat! by Denny O'Neil/Michael Golden
Batman Annual #8 (1982) - The Messiah of the Crimson Sun by Mike W. Barr/Trevor Von Eeden
Detective Comics #567 (1986) - The Night of Thanks but No Thanks! by Harlan Ellison/Gene Colan
Ms. Tree Quarterly #1 (1990) - The Name by Denny O'Neil/Mike Grill
Batman #400 (1986) - Resurrection Night! by Doug Moench/Steve Lightle/George Perez/Paris Cullins/Bill Sienkiewicz/Art Adams/Tom Sutton/Steve Leialoha/Joe Kubert/Ken Steacy/Rick Leonardi/Brian Bolland
Batman #401 (1986) - A Bird In the Hand by Barbara Randall/Trevor Von Eeden
DC Special Series #15 Batman Spectacular (1978) - Hang the Batman by David V. Reed/Mike Nasser
Batman #355 (1983) - Never Scratch A Cat by Gerry Conway/Don Newton
Batman #345 (1982) - Calling Doctor Death by Gerry Conway/Gene Colan/Klaus Janson
Detective Comics #512 (1982) - The Fatal Prescription of Doctor Death by Gerry Conway/Gene Colan/Klaus Janson
Batman #350 (1982) - Nightmare in Crimson! by Gerry Conway/Gene Colan
Batman #351 (1982) - What Stalks the Gotham Night? by Gerry Conway/Gene Colan
Detective Comics #519 (1982) - Like a Dreadnought in the Sky! by Gerry Conway/Don Newton