"Apple Tigger X"
Seriously though, I don't see why Linux is so secure (not saying it isn't, I just don't get how/why) - it's open source, any hacker can take it apart and break bits of it.
Windows Vista is pretty damn good, as I've been running it without AV or firewall for weeks and haven't picked up anything at all. Closed source means it's harder to find holes.
And "No OS" definitely doesn't count. Without any sort of OS, your computer is just a box filled with electronic components, complete junk.