I don't think there is one quality that is most important. while I feel it's important to have things in common, I'm not sure that there is a "perfect match"... Nor am I looking for a carbon-copy of myself. Everyone is different in someway. I think that's a good thing, since I think it's the differences in each other that strengthens the dynamics of a relationship. Having said that, You need to have trust understanding and at times compromise... Or you have nothing.