My picks for budget primes:
105 2.5 (any version, they're all the same)
50 1.8 series E (criminally underrated, fantastically cheap)
28 2.8 AI (underrated because of the highly regarded ais)
the next level:
50 1.2 AIS (favorite lens, any system)
35 1.4
180 2.8 - image quality out of this world