AVENGERS #25 (February 1966) was the first issue I ever bought.
AVENGERS #3 (January 1964) is the oldest issue I currently own.
AVENGERS #61 (February 1969) is my all-time favorite single issue. Story by Roy Thomas (my favorite AVENGERS writer) and art by John Buscema (my favorite AVENGERS artist). A one-time only line-up of Hawkeye, The Vision, The Black Panther, The Black Knight and the short lived super hero version of Dr. Strange.
The multi-issue epic, The Kree-Skrull War (with art by Neal Adams and others) is my all-time favorite Avengers storyline.
THE AVENGERS BATTLE THE EARTH WRECKER, a Bantam paperback by Otto Binder, is my favorite AVENGERS collectible. And yeah, I've got this one also:
CAPTAIN AMERICA: THE GREAT GOLD STEAL a Bantam paperback by Ted White.