Dear friend
the translation should sound like " Sei la cosa più bella che io abbia mai visto" or " Sei la cosa più bella che io abbia mai incontrato".
I would like to help you further by saying that "cosa" and "più bella" may as well be inverted ( i.e., "più bella cosa" = "cosa più bella")
Moreover, in Italian we also use the following exspression " Sei la cosa più bella del mondo" ( you are the most beautiful thing in the world) is of common use when referred to a lover.
Last but not least, there are at least two love songs that I can remember:
- "Più bella cosa non c'è" by Eros Ramazzotti (There not a thing more beautiful than you are)
- "Sei la più bella del mondo" by RAF ( You're the most beautiful in the world )
which you may want to listen to.
bye
Enrico
