It's a very, very personal thing. There is no 'best'. Here's an explanation why:
Real racers see almost 180 degrees forward, and at the same time, stuff is very large (real life scale, even) and detailed.
A monitor takes up no where near 180 degrees of your vision. More like 25-35 degrees maybe (very individual, the further away you sit, the smaller the screen etc). And in this small space, you must choose what to fit.
A higher fov:
+ Makes you see more to the sides (like real life)
+ Stronger sensation of speed
- Stuff is much, much, muuuuch smaller in scale than real life (massive loss of details). It really feels like just watching something go on on a TV screen rather than being there.
- The view is distorted (fish eye view, or similar effect to watching through the wrong end of binoculars)
- Due to the scale being so off, distances are much harder than real life to estimate.
A lower fov:
+ Can (not in Race 07 which has a cap at '80%' whatever that is, but theoretically) be put to a 1:1 ratio of your viewscreen, making stuff appear exactly in the same scale like in real life.
+ Translates better into real world driving due to more correct scale (heard lots of this from real world racers & simracers) - the tracks look and feel correct.
+ Better detail
+ - Becomes essentially like driving a car through a window in front of you
- blind to what goes on next to your car (you can turn your head, of course, and use mirrors)
- Less sensation of speed.
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In my own opinion, I try to have the biggest screen possible and sit as close as possible, then turn the fov down quite a bit to make it look something like real life. Stuff really 'pops' up in front of me and I relate so well to my real world car experiences. A very nice sense of realism. I have a 24" 16:10 monitor that I sit a bit less than a meter from with my head. In "Live for Speed" I find that 57 degrees FOV is a very good compromise.
And as you can see, having a bigger screen does in no way mean you should raise the FOV to see 'more'. In fact, it offers a chance to see things more to real world scale and have it start acting like a window into the game world rather than merely a TV screen.