I don't think there's just one - but I think there are only really "a few".
As Green Witch so rightly says, what we look for changes with age and experience.
As time goes by there have to be different levels of "right for each other". Julian Barnes, in his book "Talking it Over" indicated that there are three requirements:
Love
Fancy (rather British term for the lust aspect!)
Respect (I take this to include those practical/value-based things which allow us to live in harmony)
Two out of three is good going and could hold a relationship together for some time but really all three are required.
Nothing better than the hope that you may have found all three!
KP