I have never had an empty flight, however two years ago me and my fiancé boarded a flight from TFS to BHX (Monarch A321). It was an evening Christmas Eve departure back into Brum and we were 2 of 30 others who boarded the flight.
Safe to say it was the most pleasant, relaxing and spacious flight I have ever endured.
I post this as I am flying across the Atlantic on a packed United B757 from BHX!!