What drives your decisions: Price, handling/weight, ultimate performance/sharpness?

I've had a 28/2 AI lens, and sold it. I prefer the 28/2.8 AIS, which I bought for $30 used in great condition on a visit to Saskatoon. The fact that it focuses much closer swayed me, but in your case you could be more interested in the low light possibilities. Both are very fine lenses that I would not hesitate to use, but the 28/2.8 is sharper to my eye. If you are also into digital, the 24's (f/2 and f/2.8) are reputed to perform less well on digital than on film, where the 28/2.8 AIS is a very notable exception. Even though 28 mm primes are not that in favour nowadays, the 28 AIS is so good and useful that it bucks the trend of dropping prices. The better value for film only might be with one of the 24's.

I think the 20/3.5 is not a bad lens, but the 20/2.8 is better no doubt. I find my 20/3.5 more useful for IR work on digital, and for close-ups with the K1(?) ring - hope I have the number correct, it's the very flat extension ring without an aperture lever coupling. I also have a 14-24 which is what I now use on both film and digital cameras, unless I want to use filters on B&W film, where the 20/3.5 again comes in handy.

The 105/2.5 AI or AIS is a wonderful lens, especially the one with large rear element. The earlier design with small rear element is not quite the same. I've got the pre-AI 50/1.4 (AI converted), and haven't really considered the 50/2 that useful, but I have never heard bad things about it. The 50/1.4 is a great lens stopped down to f/2.8 or so. Wide open it lacks contrast, but is much better even one stop down at f/2.

David Ruether has a list of Nikkors with plenty of useful info: http://www.donferrario.com/ruether/slemn.html. The evaluations are subjective, but in a sense worth more than sterile lab tests, especially for film use. If you shoot B&W film, then almost all Nikkors ever made will be mostly good enough for all but the most strenuous requirements. If you want to do colour work and scan at high resolution, you should think a bit more about what you get.

Other lenses I can also wholeheartedly recommend:

75-150/3.5 Series E
25-50/4 AIS
55/2.8 AIS Micro
105/2.8 AIS Micro
135/2.8 AIS or Series E
200/4 AIS

I have used many other MF (and AF) Nikkors too, and most of them are good to great. The one I don't like at all is the 28/2.8 AI that does not focus to 0.2 m, but to 0.3 m instead. Very soft and dull.