If you want a secure OS with a methodical, stable release program, then you should look no further than BSD. FreeBSD for the desktop, and OpenBSD for the utterly paranoid.
If, on the other hand you want an OS hobbled together by hundreds of coders, each keen to stamp their mark on a particular library or application, get Linux. It does get the job done (Although it still runs slower than BSD in my experience:P)
We may all laugh about windows being unsecure, but imagine being able to read the root password from any user account in a PLAIN TEXT FILE