The only OS I can think of having really fond memories of was Windows Server 2003, because I never actually used Windows XP on any machine just that. It had a leaner install, better performance and all of the server OS benefits. My Windows Server 2008 R2 box was running for 12-13 years without any issues so that was nice, but I'm pretty sure anything else taken care of would have as well. Generally speaking so long as the OS works for me I don't worry about it all that much.