Formal:
* Je t'aime, mon ange. (This is the most literal translation, but sounds a bit formal.)
* Je t'aime, ange. (More poetic, but still formal)
* Mon ange, je t'aime. (Slightly more intimate, but still formal)
Informal/Romantic:
* Je t'adore, mon ange. (This means "I adore you, my angel" and is more passionate than "Je t'aime")
* Je t'aime, mon petit ange. (Means "I love you, my little angel" and is very affectionate)
* Mon ange, tu es tout pour moi. (This means "My angel, you are everything to me" and is very romantic)
Other options:
* Tu es mon ange. (You are my angel)
* Je t'aime plus que tout, mon ange. (I love you more than anything, my angel)
The best option depends on the context and your relationship with the person you are speaking to.