In my opinion: OpenBSD - Focused on security above all else. Host project of OpenSSH and pf/carp/altq?. Lags behind on architecture support and performance (Previously poor SMP performance?). Primary use: Router/firewall.
FreeBSD - All around features and performance (zfs,dtrace,pf,linux syscall emulation layer). The most popular, and I believe has the most development effort. Popular freebsd derivatives: pfSense (firewall appliance), FreeNAS (zfs storage appliance), PC-BSD (packaged up for an easier desktop experience). Primary use: Desktop/server/firewall/storage/database.
NetBSD: Focused on architecture support. Supports 57 platforms/15 processor architectures: http://www.netbsd.org/ports/. Primary use: support on embedded/uncommon hardware.
DragonFlyBSD: Interesting technologies being developed: HAMMERfs (compare with zfs), application snapshots, virtualized kernel. Matt Dillon's fork of FreeBSD adding the spirit of AmigaOS. Primary use: Compute clusters?
NetBSD is the one whose source you should study. The emphasis is on correctness and portability. The book Code Reading by Diomidis Spinellis used mainly examples from NetBSD even though he is a FreeBSD developer, because (he implies) the code is easier to read.
OpenBSD forked first and developed a reason to exist second; its storied past is really quite an interesting read, but won't leave you feeling like it is being developed by the greatest minds in security for exactly that purpose. I think its secure reputation is more fiat than fact and I'd like to see recent benchmarks substantiating the notion that it is better to run on routers and whatnot. I have no use for it.
Why not OpenBSD?
http://en.wikipedia.org/wiki/Comparison_of_BSD_operating_sys...