It's certainly not the F50.
The best looking Ferrari on the Market now is definatly the 360 Modena, followed by the 456M GT. The 575M Maranello is beatiful as well. Sh*t, every Ferrari is beautiful. I can't think of anyone who comes even close to designing better looking cars.
I think the best looking classic ferrari is either the 1984 GTO or the 1964 250 LM. It's too hard to pick with Ferraris.