The middle versions seem to be the best ones somehow. Sire, the cooperative multitasking model of Mac OS 9 was a bit, crappy, but it was really reliable if you went to work at it from a task-oriented perspective. Same for Windows 2000, more stable than Windows XP for some reason.
For Mac OS X it was 10.3 to 10.5 that was the (seemingly) most stable.
I think the scope and size of hardware, software and interaction has just gone so large that it's not commercially viable anymore to test or QA the entire system to the same standard. It's also why service managers came along, we just assume that everything breaks and hope restarting it makes it go away...
My recollection of (Apple's) System 7 is that it really got bitten by the rise of programs running dynamic code (i.e. Netscape). When everyone was operating on relatively pre-vetted apps, which had their buffer overflows cleaned up, it played pretty nice, but when that wasn't the case, it really blew up.
System 7.5 was rather worse-off than 7.0, but it got absolutely slammed because it was the first major mac OS that got saddled with web browsing, and the OS was just not fit to handle apps that had buffer overflows.
That was the big thing about those mac OSes; since they had no memory protection, if any app on the system buffer overflowed, it'd crash everything. It was fine in an environment where you really were only running one or two apps that were known to be really rock-solid, but it wasn't able to police misbehaving apps.
For Mac OS X it was 10.3 to 10.5 that was the (seemingly) most stable.
I think the scope and size of hardware, software and interaction has just gone so large that it's not commercially viable anymore to test or QA the entire system to the same standard. It's also why service managers came along, we just assume that everything breaks and hope restarting it makes it go away...