Here they are, the top five Zero Hour crossover issues (this is a lot like getting water from a stone, by the by)
HONORABLE MENTION: Guy Gardner: Warrior #24 - neat time-travel story, introduced Lady Blackhawk into our timeline (who is still around, as a member of Birds of Prey)
5. Superman: Man of Steel #37
People forget, but at the time, Superman and Batman rarely talked to each other, so this issue was significant, because it gave both characters a chance to catch up after all the changes in their lives (Batman's back breaking, but healing and returning to displace Azrael as Batman and Superman dying and returning).
Also, what a cool Bog cover!!
4. Adventures of Superman #516
Karl Kesel, more so than any other writer, seemed to really embrace the Zero Hour concept. In this issue, he uses the concept of shifting timelines to introduce a world where Rome never fell, and Metropolis is protected by the Alpha Centurion, who eventually ended up showing up in the Superman titles, and was a pretty cool character, to be honest.
3. Superboy #8
Straightforward concept, but it worked so well.
Superboy (the clone) meets up with Earth-One Superboy, Clark Kent. It was handled very well by Kesel, who uses the comparisons between the two to really develop the clone as a character.
3. Flash #94
This was a tough one. It really doesn't tie in THAT much, but it DOES tie-in, and it is part three of an excellent story (that introduced Impulse), so I can't help but place it high on this list, as it IS a very good comic.
1. Robin #10
Straightforward concept, but it worked even better than the Superboy issue.
Dick and Tim teaming up - SUCH a cool idea. And Dixon and Grummett do a good job. And what a cool Grummett cover!
That's my picks! How about you folks?