For this task, I would recommend OpenBSD. It is secure, very stable, fairly easy, and uses a KISS "it just works" approach.
FreeBSD will also work fine.
Personal preferences and opinions on this subject may vary though, coming from Linux I'm sure you're familiar with the "My foo is better than your bar" type of discussions
The OpenBSD FAQ is an
extremely valuable resource for people new to the system:
http://openbsd.org/faq/index.html
As is the PF user's gude:
http://openbsd.org/faq/pf/index.html