With 852 Votes (39.32%), Steve Rogers' emotional goodbye to Peggy Carter as he is forced to sacrifice himself (for all intents and purposes) in order to save innocent lives comes out on top. You could argue this is more tragic than romantic, but given the context and that final exchange about being late for their date, I think it qualifies. Coming in second was that iconic upside-down kiss from Spider-Man with 559 Votes (25.80%), and third was another classic scene, that nighttime flight from Superman: The Movie with 278 Votes (12.83%).
Here is a full list of the results..