On the subject of lighting, playing FH3 in HDR is for me the most impressive Forza lighting, because it has a completely different appearance from the lighting you get in FH4/FH5.
Based on my experience travelling around the globe, it really is true that the quality of the ambient light changes depending on where you are in terms of latitude, and FH3 shows that difference well. So for me it has probably the best visuals in terms of lighting - but you can only really demonstrate this on a proper HDR screen.
Oh, that's another good point: FH1/FH2 don't have HDR. In comparison, FH5 without HDR (I tried this once on a non-HDR PC screen) looks really flat and dull. HDR elevates the visuals hugely.