OpenBSD is the most secure OS on earth including windows, Mac OS ,Linux and other BSDs or any OS ever made. If you think it must be backed by some large organization, then you are wrong. It is a complete volunteer based OS with its latest release OpenBSD 4.6 on Oct 18, 2009. It is the OS that openly challenge crackers.
Two things have always been paramount in its design: security and open source. These two have also cause much controversy as OpenBSD don't include any proprietary software. Also they are opposed to not making code available for commercial works unlike GPL. Apache version 1.39 is used but they have modified it.

Its slogan is:
Only two remote holes in the default install, in a heck of a long time!

It must be noted one of the bugs above affected all OS using openssh and in second bug you can't control just do dos attack. Both have been resolved.

No OS have that record.
Any program that runs on Solaris, FreeBSD, Linux, BSD/OS, SunOS and HP-UX in turns can be run on it.
You don't get much on default install but major cause of security vulnerability is running thousand services which you don't need.
BSd would have won and beaten windows easily but a lawsuit by AT&T which was dropped when Novell bought the source code of UNIX from former.Check this out:
Is Linux For Losers? - Forbes.com