Formal:
* דו ביסט מיין ליבע (Du bist mein libe) - "You are my love" (literally translates to "You are my beloved")
* דו ביסט מיין האַרץ (Du bist mein hartz) - "You are my heart"
Informal:
* דו ביסט מיין קעניג/קעניגן (Du bist mein kenig/kenigin) - "You are my king/queen" (This is a very affectionate way to say "You are my love")
* איך האב דיר ליב (Ich hob dir lib) - "I love you"
The best way to express your love in Yiddish depends on the context and your relationship with the person you're speaking to.
If you want to get creative, you could also use a quote from a Yiddish poem or song that expresses love.