My overall favorite OS was OS/2 Warp. It was incredibly stable and had many features that were way ahead of it's time. It included Windows 3.1 and DOS compatibility, so it could run most of the PC applications that were available at the time.

At this point, I stick with M$ OSs because of their wide spread adoption. If 80% of the world is using the same OS, then that's probably where the jobs are.

However, I agree with others who say that it depends on your requirements...

Price
Performance
Capacity
Support
Stability
Flexibility
Application availability
Recoverability
Scalability
Security
Monitoring
Form factor
Connectivity
etc...

Each OS has its strengths and weaknesses in these areas and many others. It's more appropriate to discuss OS strengths and weaknesses in specific areas than to ask flat out which is better.