Rosenfall
Which Crisis, the first or the second?
Actually, I think according to DC there's been four events which could be considered as Crises:
1. The end of the Golden Age and the subsequent relaunching of characters at the start of the Silver Age. (Not a crisis as such but certainly a rebooting of the DC universe.)
2. Crisis on Infinite Earths
3. Zero Hour
4. Infinite Crisis
If the first one had never happened, that would be very interesting. No Justice League, no Green Lantern Corps, no multiverse... in 1956, when they decided they'd bring back the Flash as a different guy with a different background, I imagine they had no idea of the eventual repercussions.