Originally Posted by flavio81
My favorite Nikkors are the pre-AI series and all the classics have already been mentioned. Particularly if you value "bokeh" and a look full of "character", Nikkor pre-AI is what you want.
Seriously though, there haven't been that many changes to Nikkor primes in the AI era (which only lasted four years) - and some changes they made were improvements (the return of the 20/3.5 after the K era 20/4 interlude), while most were invisible in the image quality. The notable one exception downward was the switch from 85/1.8 to 85/2 between K and AI (they only made up for that with the - more expensive - AI-S 85/1.4).
They upgraded more lens designs when they switched from F/C to K type (which probably went along with automating their assembling lines), but even there, it is hard to make out IQ changes between before and after. Overall, Nikon primes have been relatively static - the majority of them never were redesigned and only ever went through the series of coatings Nikon used as standard. Like most makers, Nikon usually introduced a faster lens rather than update their existing speed (but more of that happened in the much longer AI-S era) - the big roll-up of redesigns only happened when lenses were electronified for AF.