If one is really concerned about security and an OS that has extensive review for security holes, I'd look at OpenBSD.

In the Unix world, I look at the two major camps (Linux and BSD) like this:

- Linux adopts new ideas and features faster, but it is less stable, and the various distro's tend to have lots of overlapping forks (in terms of function/hardware).

- The BSD's (FreeBSD, NetBSD, OpenBSD) are all more stable, move at a more measured pace, have very different ideas of managing the development team than Linux overall. I'm not saying that this is superior or inferior to Linux, it is just what it is. When you need new hardware supported "now," Linux might be the best play. When you need stability, BSD's might be your best play.

The older I get, the more I value stability. 

- OpenBSD is probably the most secure, and has had only two remotely-exploitable vulnerabilities in the default installation in a pretty long time.

- NetBSD would be my choice in small, embedded  systems. It is most configurable to section off stuff you might not need in an embedded system. NetBSD also has been ported to more hardware chips/platforms than the other two.

- FreeBSD would be my choice if I were wanting to run a large, SMP server and I needed performance for many processes/users to scale.

My biggest issue with Linux is that they have this, I dunno how to describe it Dave ... it comes across as a mixture of inferiority complex and quixotic agenda of taking on Windows in terms of attracting desktop users. It's something that I haven't completely understood going back to the late 90's. It doesn't come from Linus (whom I've met, BTW, nice guy, doesn't suffer fools), but it seems to be this  recurrent agenda that  pops out  at times. I guess I can understand the desire to gain market penetration, but after years of using mainframe/supermini/mini and small CPU OS's, when I want Unix, I want Unix. I don't really care for a Unix that comes up with bizarre attempts at a GUI overlaid on top of X Windows or some such. This is where OS X is clearly superior - they ditched the idea of using X Windows for their window manager, and went with something that was  more like Display Postscript. As a Unix with a slick GUI, OS X is clearly superior to all of them. OS X looks and smells like BSD from the User UI and user-space API perspective, but down in the guts of the system, it is a microkernel which has some rather slick security attributes.

The net:net is that security doesn't come quickly, cheaply or easily. It takes eyeballs looking at and reviewing code, and it takes real thinking about design and implementation, and in the future, I think it will require ditching C/C++ as an implementation language.

Sorry, I've been prattling for awhile here... this is all deeply "inside baseball" nonsense about Unix...