Depends on how you define "best". If you mean best "for its time", then the answer is DOS. Not even close.
Best recent OS that will run modern software and games, 98SE (clean install "for pcs without windows" version).
In my opinion:
DOS - Always worked, never crashed, simple to use, and very precisely written (the last Non-bloatware OS - hmm, wasn't this the one that BG sold to IBM, and then bought from some poor ba$tard for 50k? hehe).
Win 3.1(and 3.11) - slow, buggy, annoying, and overall non-user-friendly - at this point, the MAC version where it came from was probably better, but MS did a much better marketing job.
Win 95 - buggy, and no USB support, not even via a patch (well, not officially)
Win 95 OSR 2.1 - less buggy, and added USB Support (sort of - via a patch, but you have to buy the whole new OEM OS to get the patch - officially)
Win 98 - Good USB Support (with a whole bunch of new bugs)
Win 98SE - Bug Fix for Win 98 - reasonably stable (for a MS OS)
Win ME - rebadged recompiled version of the 9x kernel with a whole new set of bugs, and they took away my DOS window - pass
Nack