Secure OS

Nowadays, probably the most secure OS are hardened Linux systems (like hardened gentoo and RHEL), Trustes Solaris 11, OpenBSD, QubesOS, Novell NetWare and OpenVMS.
What do you thing is doing the best security approach of this OS.
And what do you thing is better in practise?
(You can also include other OS you consider secure)
And what do you think os security on microkernel systems?

>>1
And what do you think os security on microkernel systems?
Microkernels are the only ones small enough to be formally verified (eg. seL4). However, that only covers the kernel itself. There's no guarantee the servers needed to implement a fully functional operating systems are secure or bug-free.