Here's the answer:
“あなたよりも私のほうが彼を愛しています”
(anata yorimo watashi no hō ga kare o aishi-te i-masu)
Watch a real native speaker say it:
Here's how it sounds in a textbook:
You could also say:
- あなたよりも私の方が彼を愛してます
Fun fact: the hundreds of thousands of people learning Japanese with Memrise get this phrase correct 92.08% of the time!