Definitely System 7.1. Later versions seemed to be the compilation of more and more features that were useful to fewer and fewer people, with no real breakthroughs like preemtive multitasking or memory protection to ensure that software worked better. System 7.1 also worked with a huge range of software, and on a large number of models.

System 6.0.8 is a definite runner-up. The memory efficiency and relative speed of the system software is a refreshing change from more modern systems. There is also a lot of excellent software that runs under System 6, and most of that software reflects that philosophy.