I think that the best compliment/message that you could ever give anyone is "you are worth my time." This is because, in every instant, you are either moving closer to someone or farther from someone. If you choose to spend time and move closer to one, you are naturally growing apart from another. Time is precious, thus its use to get to know one person over many others is the highest of honors that you could pay someone.
Now, considering that it is a completely artless thing to say to someone, it cannot realistically be used as a complement/flattery. It would take a long winded explination like this one to make the person understand it. It is just the highest complement that you can give to someone, not necessarily one I would tell someone (and besides, every time you choose to spend time with someone, you are saying it silently). Unless the other person is a fan of obscure and long-winded compliments, like in As Good As It Gets, where Nicholson says, "You make me want to be a better man," and then has to go on and explain for 5 mins what he means :tongue: