For me it's OS9.

I switched over during 8.6 and was really happy when OS 9 came out. I love the platinum theme, spatial finder and the ease of use throughout the OS. If OS 9 had a decent browser and support for auto memory allocation it would be perfect for me. That being said I still use my 8500 and BeigeG3MT on a weekly basis.

Some things are just easier on a Mac.