The developers on NetBSD are a great bunch (not that the other BSDs don't have great developers), and for any sort of a server or desktop it's my first choice - for embedded platforms it's my only choice. 2.0 has some really great features that have, in typical NetBSD fashion, been a long time coming, but now that they're here, they're REALLY well designed.
FreeBSD is also a good one to try. I don't like it as much, but that's mostly just personal preference. DragonFly looks interesting, but I haven't bothered with it yet. OpenBSD is, well, rudimentary at best - I've only ever encountered one thing it does that the others don't do (yet), and that's RFC2385 support, which I highly doubt you will care about. Other than that, it's crude, problematic, and mostly hype - NetBSD and FreeBSD are every bit as secure, possibly moreso.
Slashdot Mirror
FreeBSD is also a good one to try. I don't like it as much, but that's mostly just personal preference. DragonFly looks interesting, but I haven't bothered with it yet. OpenBSD is, well, rudimentary at best - I've only ever encountered one thing it does that the others don't do (yet), and that's RFC2385 support, which I highly doubt you will care about. Other than that, it's crude, problematic, and mostly hype - NetBSD and FreeBSD are every bit as secure, possibly moreso.