The El-Nikkors are consistently extremely good, yet quite cheap. TBH, you can't really go wrong with any of them so just pick a focal length that suits the film you are shooting, ie 50mm-63mm for 35mm film, 75mm-105mm for medium format film.
If you have bottom less pockets then consider the Leitz Focotars or any of the Rodenstock or Schneider APO lenses, but, you won't gain that much, and most of them will cost a bomb.
If you can't afford the 'better' El-Nikkors, the 63/2.8 N, 63/3.5, 80/5.6 N and 105/5.6 N lenses are not cheap, then look at the 50/4 or 50/2.8 N or the 75/4 N. The old 75/4 (not the 'N') is an excellent lens and is very cheap.
Generally the later the lens you buy the better it will be, generally. For example, there are lots of Schneider and Rodenstock lenses that date to the 60's-70's that are quite cheap and will work well, but typically the latest version of any of those will perform better than the earlier versions. There's also not a massive difference in prices between them so I would always try to buy the latest, such as a Schneider Componon-s vs an earlier Componon, or the El-Nikkor 'N' over an older El-Nikkor (there are exceptions).
