Yes, the age old battle. Clearly, I support Linux over everything, but oh well.
Windows
This is the easiest OS to use, IMO, closely followed by MacOS X. However, it is integrated with programs that have security holes and do not work as well as their alternatives (IE vs Firefox, Media Player vs WinAMP). Also, Microsoft seems to be working on adding features (which won't be used by most users) instead of fixing security holes.
Linux/UNIX
This is my personal favorite. Linux/UNIX is the perhaps the hardest OS to use, but for power-users suck as myself, it comes rather easy. Something to install on your family computer and to expect your parents to understand and use? Not really. However, the upsides. First off, its pretty powerful. Windows has a bunch of limitations on it so that users don't screw themselves over (system files can't be deleted, raw sockets cannot be used, etc), whereas in Linux, you can remove/rename files while they're loaded (automatically upgrading programs), along with many other things. Also, Linux/UNIX is used by a very small portion of the general computer users, thus is a tiny target for malicious software development, and even if software was written for it, as long as you're smart and not running as a root administrator, you can do very little to harm your box. I'm not sure about the UNIX/BSD distrobutions, but Linux is totally open. The source code, suggestions, contribution, and even the tech support community. If there is something you don't like about it, you are totally free to change it, and even suggest that your new code be added to the program so that everyone can use it. As for the community, I idle in #ubuntu (my distrobution) on irc.freenode.new and help whenever I get bored.
MacOS Classic
I don't have much experience with this, but from what I've used it for, it was both stable, secure (many more people use Windows than MacOS Classic, making it a smaller target for virii), and easy to use.
MacOS X
As very few people know, MacOS X is basically just another (high priced =/) UNIX distrobution. I have never been able to bring this to a fatal error (except in booting, stupid hard drive). After I was introduced to Linux, I had already switched back to x86, but I'm interested in installing this on cave again and playing around with it. Basically, this is the stability backend of Linux/UNIX rolled together with the easy-to-use frontend of Windows (ease of use wise, not design).
Well, theres what I have to say. Go ahead and add to it.