Last month marked the release of JLA/Avengers 1 drawn by the late and great George Perez(may God bless him) and written by Kurt Busiek.
This crossover not only was the biggest Love letter for all super hero sagas but also the Hallmark of the big 2 after the Comic Crash of the 90's, a crossover on which is remembered by having not only some of the best stories of how DC and Marvel would work together but also a sendoff of a age long gone and something that sadly will never happen these days due to events following after it was done.
Also it made possibly the best and still the most memorable cover art of all comic books history, personally the top 1 of all, which is Superman's Last Stand at the end wielding Captain America's shield and Mjolnir with both Multiverses clashing as the Crisis is at its peak.
Recently i got myself to read all over it again and i must say, it aged well.
Also, some people say that the events of this comic influenced what would happen after during the remaining of the 2000's for both sides, though i don't think this is possible.
Though it is still "canon" that all heroes and villains from both sides still has memories of what happened, though they don't mention at all.