I'd say that there's a "best OS" for each computer/use.
desktops:
If you have a 486, go for MSDOS and install Windows 3.1 on top.
If you have a fast 486 or a Pentium, check Windows 95 OSR2 or Windows NT 4
If you have a Pentium MMX, AMD K6 or a Pentium II machine with less than 500 Mhz and/or 128 Mb RAM, Windows 98SE is your OS.
If you have a Pentium III or AMD K6/2 with more than 500 Mhz and at least 192 Mb go for Windows 2000 Pro SP4.
If your computer goes above 800 Mhz and 256 Mb RAM, Windows XP Pro SP2 is your best OS in the Windows range.
If you have a 64 bits processor, Windows XP 64 bits.
servers:
For old machines you better get Debian and a headache
For machines in the range of 733 MHz to 1 GHz I'd suggest the Fedora Core Linux distro. It works like silk and I've found it to be the easier Linux distro to deal with.
If you have a powerful server and still want to go for Windows (Linux or FreeBSD would be better for this), Windows 2003 server is neat!