CentOS at work because of the RedHat/security/enterprise idea. I agree that it seems more locked-down by default than Debian etc, and the lack of installed packages makes it nice and compact and leaves less potential entrances. It also makes it a bit more of a pain, though.
Debian or Ubuntu when given the choice myself - I'm not sure why, it just seems to agree with me better than CentOS. CentOS doesn't do anything wrong, I just don't click with it - it does things I'm not expecting whereas Debian seems to make more sense.
I'm going to try Arch at some point, but I've never really got what it's differentiator is meant to be - what does it do that CentOS/Debian/Ubuntu don't? I've tried Mint but never got it working properly, and Knoppix a few years ago as my first linux experiment - but it seems to have dropped away a little now that nearly every Distro has liveCD capability.