This is easy for me as it is one of my all-time favourites. 250S.
... or the Alfa, of course.

In agreement...so, so beautiful!

This might be my absolute favorite of these. Just beautiful pictures.

My favorite one though... hm...
The alfa picture is stunning, the ferraris are great in any color, but I think I really like that 911 in burgundy. I haven't seen one that color before and it might be my new favorite for them.

Different cars look good in different colors obviously, but in general I'd have to say that my favorite shade of red would have to be burgundy or bordeaux (claret?) or something similar.

However, it is absolutely impossible to pick a favorite of the cars above. All I can say is that it's not the Tatra or the Cadillac.

That Fiat 8V Supersonic takes the cake for me.

I also though it was a Fiat 8V, but it turns out two Supersonics were based on the Jaguar XK120. One of which is depicted here. It is however uncannily similar to an 8V.

I will always lean towards the understated beauty of dark red. Of course there will always be exceptions, but bright red has always been that little bit too much. With the deeper version you get the 'passion' without the misunderstanding that you might not know anything about the car you're driving! (A crude generalisation I know)
On a side not, I have always been particularly fond of a pale bule exterior with burgundy interior. A classic combination.