[MrDrEsq]

still would avoid using あなた even among friends.
anyway, olsson88- did some asking around today and came up with this.
you have two translations
translation 1) 最も大切なのはあんたの幸せ
motto taisetsunanowa anata no shiawase
translation 2) 一番大切なことはあなたの幸せ
ichiban taisetsuna koto wa anatano shiawase

translation two, i changed the mottomo to ichiban as the vast consensus was that it sounded more natural. mind you, both translations are correct grammatically and mean the exact same thing. now, when asked to produce the same translation for "the most important thing is your happiness", all the exchange students produced translation 2 with little to no differentiation. When asked which they would prefer in a letter, all Japanese picked translation two as being easier to understand- HOWEVER, when asked what they would write to express the same feeling, the Japanese came up with something like this:

わたしはあなたが幸せになることが一番嬉しい
watashi wa anata ga shiawase ni naru koto ga ichiban ureshii

so anyway,
there you have it. my endorsement goes to the japanese's 文.