There should be all 3 in a relationship/marriage.
I'm going to go with Love, because with true love there will be truth and sex is a natural expression of love.
yes all 3 are important but i choose truth. i don't think you can really love someone unless you trust them enough that they won't lie to you. so truth is a building block of love and love will crumble without truth.
As an aside, truth and honesty are indeed lovers, however, they have an open relationship .
The greatest of these is...love!
All three, they are all needed.
†Jinxy Approved†