"Is it getting hot in here? Or is it just you?"
"Did i just die and go to heaven? Cause you must be an angel."
Both of those are proven to work at least 70% of the time.
Others:
"Quick! someone call the cops.Because you stole my heart."
"I've seen allot of hot people, you're the hottest."
"Someone get a bucket! Because you're on fire."
"I bought some beautiful flowers, but they weren't as beautiful as you."