First, I should note that this type of crossover is the one I defined here as the STATIC CROSSOVER:
In such works, the author assumes an overall cosmos in which all of the myth-characters he invokes are capable of encountering one another at any time.
The first example of the static type that I cited on this blog was FANTASTIC FOUR ANNUAL #3, Marvel Comics' first large-scale assemblage of most of their 20th-century characters in one story. CRISIS ON INFINTE EARTHS-- "COIE" for short-- has many of the same faults and virtues of this annual. On the one hand, the seasoned fan enjoys the experience of seeing characters mixed together that did not typically meet, be it Mr. Hyde and Hawkeye, or Swamp Thing and the Losers. On the other hand, the meetings are so short that there's often a sense of frustration in such brief encounters.
A lot of COIE consists of characters looking up at the sky in apprehension or laying plans to deal with the Anti-Monitor, and these don't exactly give artist George Perez the chance to excel with his mastery of superhero kinetics.
The best issues to seek out for heroic action in the Mighty Perez Manner are COIE #6, in which the DC heroes contend with characters from other publishers, including Fawcett and Charlton:
And COIE #9, which puts the heroes in conflict with a contingent of DC's best villains, plus a tiny handful of Fawcett villains.