Well, I like to break it down like that for US:
1. Mercury / Gemini era
2. Apollo era
3. Space shuttle / International Space Station era
4. What's next!
For russian, the technology havent changed alot over the years so.
I also like the idea of manned versus unmanned space vehicle. Specially if you want to integrate satellites, telescopes and other deep space exploration robotised spacecraft.
If so, sorting them by country and then by manned or unmanned would the way to go IMHO.
No matter where you go ....... there you are.