HONESTY! Without it you can't have TRUST.
Everyone has their own pecking order of which traits are important to them. (looks, financial stablity, affectionate, romantic, passionate, considerate, share the same goals....etc) Without demonstrating honesty & integrity it's impossible to build trust.
No one can be truly happy with someone who is not (honest) and therefore they can't (trust) them. That would be like building a house on foundation made of sand.