Let's don't kid ourselves, shall we? Ninja Scroll is not the best anime ever, in fact, it's pretty high on the list of
the worst anime ever made; it's easy to see why nobody gave a damn about it in Japan.
What does it have going for it? Decently animated action scenes. I'll concede that much. But Rurouni Kenshin has great action scenes, and as an added bonus, an interesting plot, something that Ninja Scroll lacks. Ditto Princess Mononoke. You want action? There are better places to find it that Ninja Scroll.
The plot of Ninja Scroll is. . . wait. I don't remember much of it. That's right, because it didn't make any damn sense. Silly me.
The character designs? Pure shounen crap. Even the ninja chick wasn't particularly attractive.
All in all, not even close to the best anime ever made.