"Crisis on Multiple Earths" by Len Wein and Others
This is a reprinting of Justice League of America from the 1970s. All of these stories are from the old, pre-Crisis DC continuity, and revolve around the partnership between Earth-1's JLA and Earth 2's JSA.
The one had me feeling about 7 years old - all over again. I can remember reading this comic when I was VERY small, too small to even have an allowance. My grandmother would sometimes drive me to a bookstore in Astoria, Oregon, and I would stock up on comic books like this one. While the old JLA comics were probably only my third favorites (behind The Flash and Marvel's Conan), I retain a soft spot for these "DC team-up extravaganzas."
(Re)reading these old JLAs reminded me of something contemporary comic books lack: optimism. Yeah, they were super-corny, and usually made about as much sense as an episode of Superfriends, but there was a kind of innocence about them that you rarely find in modern comics. Flipping through this one, I always had a sense that everything would be right in the end, and that was a good feeling.