mmmm Win98 is out the door. W98 SE is good and stable. WinME is more stable, supposedly, and a halfway house to WinNT. DONT upgrade from win95/98 to winME- it runs like a dog, though a fresh install is fine. Absolutely NO DOS support on ME, but that's only of real use for diagnostics/old school boys.
The best,if you are not planning any fancy hardware in Win 2000 (W2K). Absolutely 100% rock solid, excellent memory and resource management and essential for serious trading platforms/traading.No DOS support here either.
I run W2K for my serious trading, and W98SE at home.