XP Pro.

Used Windows 3.1 briefly, DOS, Win95, 98, 98SE & 2000 Pro and all were improvements on their predecessors.

Like Robson have used them all extensively, (apart from 3.1), even to the point of becoming a certified 'Master user' in 95/98, 2000 and XP.

XP is the most stable to date, though I've had good stability since 98SE. I often have XP running for months at a time without the need for a restart.