CentOS is just a free-of-cost rehash of Redhat Enterprise and if you like that sort of thing then imo
Scientific Linux is a better option - it's essentially the same thing but more up to date and better engineered. There's nothing scientific about it, it's just called that cos it's made by the people at CERN and used to run, amongst other stuff, all the PCs at the LHC site.
Sticking with RPM based stuff, there's
OpenSUSE, which is a bit of a love it or loathe it distro. Especially when it comes to the stuff they pile on like YaST. Personally I loathe it but it's well respected and widely used. A couple of things that make it interesting are the Tumbleweed repo, which turns it into a rolling-release distro (I always use rolling releases on (not 'mission critical' stuff, obviously) servers because fuck upgrading every 6-18 months. The price of this is obviously that you have to fix stuff when it breaks. Which it will). And secondly
SUSE Studio, which allows you to, essentially, customise your SUSE in a live environment and then download the iso/vm with all your customisations intact.
Debian's obviously popular on servers and a very good choice. Especially on the testing repo (which makes it essentially a rolling release). Robust and extremely well tested and with tonnes and tonnes of documentation. Though I can't really see an argument for using Debian over Ubuntu Server on something small-scale, Ubuntu's PPAs are just too handy.
Personally I use
Manjaro, which is basically just an easier-to-install respin of
Arch. I absolutely fucking adore Arch, but installing it is a pain in the arse, hence Manjaro. Can't beat it (Arch/Manjaro) for simplicity and ease of configuration. The Arch ethos being to present all packages with as few modifications as possible - no config tools, no default configs. So everything is very standard and the
documentation is amazing. There's a bit of a learning curve at the start (which basically just involves getting it into your head to use the wiki) but once you're over that it's easier than most distros because there's only
one way (the correct way) to do each thing - you're not dealing with layers of abstraction fighting eachother. The 'down'-side of this is configuring most things means getting your hands dirty in the text files.
Summary: If you liked CentOS then I'd recommend Scientific Linux. If you didn't, I'd recommend Debian/Ubuntu. If you want something rolling-release I'd recommend Debian/SUSE/Manjaro. And if you want to learn and get better at Linux then I'd strongly recommend Manjaro/Arch.