Mac OS 9.0.4 is widely considered the pinnacle of the "Classic" Mac OS lineage. Released in April 2000, it was the final iteration of the original Macintosh operating system before Apple fully transitioned to the Unix-based Mac OS X (now macOS).
A more complex emulator that can technically run later versions but is often tested with 9.0.4 for stability. mac os 9.0 4 iso
Enthusiasts have successfully used these ISOs to run Mac OS 9 on everything from the Raspberry Pi to M1/M2 Macs via tools like UTM. Mac OS 9