50, as any other setting causes weird effects if you save custom cockpit views and then recall them (see below). But I normally zoom in a little depending on the aircraft. I don’t like wide angle - for me it’s then difficult to judge height on landing and everything looks less natural, though situational awareness is improved.
60-70 would be preferred if not for the camera bug. This on a 16:9 display with TrackIR.
Custom camera bug:
With a default zoom setting above or below 50, the camera can end up in a super zoom or super fisheye state, well beyond the normal limits. And in any case your recalled view is never quite as you set it, unless you run 50 for default.
Haven’t checked recently, but the issue was described in this old post - see last screenshot for example.