As the title suggests, this is a comparison of the f1.8, f2.5, and f3.5 versions of the 3.5cm W-Nikkors. For this comparison, as shown below I used late LTM versions of each lens (each lens dates to around 1956-8). The f1.8 and f3.5 lenses are optically pretty much perfect - no haze, fungus, or separation and only a tiny amount of dust. The f2.5 lens had haze and fungus when I got it, but it cleaned up quite well and the remaining minor damage to the glass doesn't seem to affect performance at all.
W-Nikkor 3.5cm f1.8 - seven elements in five groups modified double gauss optical formula unlike anything else at the time. For a short time it was the fastest 35mm lens available in Leica screw mount.
W-Nikkor 3.5cm f2.5 - six elements in four groups double gauss optical formula very similar to Leica's Summaron 35mm f2.8 and f3.5, and later Summicron 35mm f2 version 2/3.
W-Nikkor 3.5cm f3.5 - four elements in three groups Tessar type optical formula very similar to Leica's Elmar 35mm f3.5.

The 3.5cm W-Nikkors by jonmanjiro, on Flickr
W-Nikkor 3.5cm f1.8 - seven elements in five groups modified double gauss optical formula unlike anything else at the time. For a short time it was the fastest 35mm lens available in Leica screw mount.
W-Nikkor 3.5cm f2.5 - six elements in four groups double gauss optical formula very similar to Leica's Summaron 35mm f2.8 and f3.5, and later Summicron 35mm f2 version 2/3.
W-Nikkor 3.5cm f3.5 - four elements in three groups Tessar type optical formula very similar to Leica's Elmar 35mm f3.5.

The 3.5cm W-Nikkors by jonmanjiro, on Flickr