For me it was WinXP. I'm still using it (among all later versions of Win) since I am using programs I need for work that have never been upgraded for later Win Versions. WinXP is actually the master sys to network all computers running later systems. WinXP still proves to be the most stable system. Win7,8, and even 10 are crashing (not often of course!) WinXP doesn't. The system runs 12 hours 7 days at average. The only problem is now to not get good graphic cards anymore that would run with XP. So far I failed to find an nvdia for my system. The old graphic card gets weaker and weaker, it won't supply any more dual screen, and when it fails completely, that would be the end of my XPeriance. Very sad!