If "I am very happy that I have you as my best friend." says exactly what you want (and I'd argue that it does) why shorten it?
The declaration 'you are my best friend' actually makes the part about how you feel superlative as, one generally wouldn't make the declaration to someone that didn't make them happy. Unless, of course, one is being ironic i.e. they are not a very good friend but they are the best of a poor selection.
I don't think I've helped really, have I?
English is such a beautiful and complex language, I'm happy to admit I sometimes get it wrongly!