The ranking system is pretty neat as a way to make the levels more challenging, if you really want to go for 100s. I could never get 100s on every level in any of the games. I didn't really like how some things were tied to rankings in some games though. I also didn't like how you were basically barred from using cyber elves until... zero 4, I think? It just seemed sort of weird for the game to give you upgrades but then slap you on the wrist for using them.
I also think the Zero series has pretty good music, but not amazing. It's mostly like if X3's music were good, I think. There's a few tracks I really like but most I don't remember that well.