Or vista for that matter
Unix is the most secure to my knowledge and has the least amount of viruses and exploits written for it. I would say OS X is up there as well since it is based off of Unix (free BSD to be exact). Unix and linux are almost the same but they are not the same, if that makes any sense. they are of the same species but not the same breed. Unix has been around for a long time and Linux is kind of it's bastard step child. Since Linux is a bit newer and has other features built into it I think it makes it a bit more vulnerable, also Unix is more of a closed system where as Linux is more of an open source community type OS (even though Unix is also open source, but has stricter licensing) so if a user has more packages and more software from many different configurations and more chances of loading sloppy coded applications which gives Linux a higher chance of being exploited over Unix.
Not entirely true, by nature *nix OSes are more secure simply by design. The Kernel is a locked down layer of the OS which is not touchable by any type of application, software, driver, etc. It controls the hardware on a low level, so nothing has direct access to the kernel or the hardware. Instead processes run at certain levels and are given permissions through the shell, making it much more secure by nature. Windows is very poorly coded when comparing it to a *nix system on a security comparison.