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2e.. ultimate power is good for crunch, reorganizes things in a much more sensible way. W&W and mecha & manga contain a few good crunch ideas. The rest of the books are more about fluff instead of crunch, several of them are fairly good at that.
For 2e the most important books are the core book and Ultimate Power. If you have those, you don't really need anything more.
That said, there's a lot of crunch in both Wizards and Warriors and Mecha and Manga and some of it is pretty darn good. There's also some good stuff in Hero High although that book is certainly more slanted towards fluff. Really good fluff mind you.
Mastermind's Manual has all sorts of alternate rules to tailor the game to your liking, some of which was lifted and put into Unearthed Arcana. AFAIK, Golden Age has a mass combat system that's interesting though the book itself lends itself well to Pulp Adventures.