Respect. If you respect your partner and yourself, you will be honest and loyal.
There's nothing better than trust
Without that it'd be a very unhealthy relationship and no one will be happy
I think all three are important, but personally I think trust comes first. In order for me to respect a guy, I need to trust him. And if he respects me, I can trust him. After I can trust him it will be easier to be loyal to him.
No one likes a rude dude. If the person disrespects the significant other in front of their friends and family, they end up with nobody.
All three are equally important. :-)
All three thank you very much.
Respect, and with respect comes loyalty.