The most important thing? That would be a pretty shallow relationship requirement. It's true that people have certain opinions as to what they consider attractive, but to say that it's the "most important thing"... I can't really see a stable, lasting marriage being based on height, or on any other physical attribute for that matter.
I think the most important thing in a marriage is that the two people are compatible, comfortable and supportive of each other.
If you're asking if most women are attracted to men who are taller than them, I don't see that being very far off the mark in a lot of situations. But I don't agree that it's an absolute marriage staple.