Side Show has some great ones. "You Should Be Loved" and also "Private Conversation," IMO.
Anyway...
Carousel - "If I Loved You" - I think it's the best Broadway love song ever, tbh.
Parade - "All the Wasted Time"
The Last Five Years - "The Next Ten Minutes" - not sure if it's powerful in terms of the music, but I think it's very powerful emotionally
Songs for a New World - "I'd Give it All For You" - love when it reaches the bridge, or whatever you call it
Passion - "No One Has Ever Loved Me" - the song itself isn't powerful, but those orchestrations at the end are just breath-taking
The Secret Garden - "How Could I Ever Know" - seriously, their love transcends even death!
Miss Saigon - "The Last Night of the World" - probably one of my favorite Broadway love songs, ever
Jekyll and Hyde - "In His Eyes" - agree with the above poster
Jimmy, what are you doing here in the middle of the night? It's almost 9 PM!