[Nyororin]

As an actual translator/interpreter, I would write it as;

もっとも大切なのはあなたの幸せです。
(mottomo taisetsu na no wa anata no shiawase desu)

I see no problem at all with using anata unless it`s someone you just met on the street. But I highly doubt that is the case with this sort of phrase. Anata is NOT overly personal.