Whenever someone asks me "What Linux distribution should I use?", I tend to reply, "take a look at mirror". It's up to you, after all.
If you need typical enterprise-level boringness with not-ever-changing software versions, pick RHEL/SLES/CentOS.
If you don't need those enterprise distros but need stability (meaning not often changing software versions) and the latest-and-greatest software is not a necessity, pick Debian.
If you feel adventurous and need more current software, try Ubuntu Server.
Other options if you need more current versions and don't care about stability: FreeBSD (with its ports ... though this is usually very stable!), Arch Linux, Gentoo, Debian Sid.
But generally, use the distro you are familiar with. Use your time wisely and install also some other distros, get to know them and see how things are done -> learn something new -> apply that to your knowledge.