someone did a poll
to see which distro was best for a server similar to the one you might be setting up. now of course the results are biased towards slackware, because many people on LQ use it. i too think slackware is a good, stable, reliable distro. but you can look at the results to see the rankings people gave on this site.
now for my opinion: if you want an ultra-stable, ultra-secure OS, i'd go with OpenBSD or another *BSD. so i guess if i HAD to rank the top 5 in my opinion, it would be: OpenBSD (check them out
), FreeBSD (i personally use it), NetBSD, Slackware, and then Redhat (a lot of people on LQ seem to use this for a server, so it must be good?). BSDs are amazing. many have several year uptimes, and Yahoo! uses FreeBSD (maybe not anymore?).