From my personal experience:
98 collapses easily under pressure, and has memory leaks
NT is better, but suffers from the blue screen too often
ME no experience
2000 getting better. Can stay up for weeks at a time
XP seems to be quite reliable. Don’t have sufficient info yet
But the winner by a huge margin:
Linux (RH 7.1) Up for over a year, running a web server and Gimps in the background, so the processor was running flat out. The end failure was hardware: the building that contains the machine has terrible power, and the battery in the UPS system died.
I would never put anything that was life-mission critical, such as a signalling system on any operating system that came out of Redmond.