RedHat you have to pay for.
The free version of RedHat is either Fedora, or CentOS. CentOS being more in line with RedHat. Over them all I'd prefer to just stick with Ubuntu Server for a server OS.
Plus if one has used the Ubuntu environment for years, to include distros built from it (example, Linux Mint & many others). this makes life a bit easier than stating from an entire new platform. While I don't have a server in use (though need one), would choose the Ubuntu brand over any other.
I did play with CentOS in my spare time, though never got to the comfort zone that I've had with Linux Mint for nearly 8 years. So the best server application will vary from one person to the other & the chances will be high that if a long term Linux user, the user won't vary far from home, choosing a server distro that uses the same base distro. Really, as long as it takes to get a good feel for a distro, and many has to try a few different types to find the right one. Therefore there's no need to be thrown off in frustration in making a different based Linux server work with their installed OS (if the person is running a Linux distro as their daily OS). If the person is running Windows & wants a Linux server to save on huge licensing fees (over $500 for a single machine copy of the latest Windows Server), then the person can be more open in regards to which Linux Server distro is chosen.
If no previous Linux experience, wants to learn & be up and running as quick as possible, then I'd recommend the Ubuntu Server due to the massive amount of support available in articles, tutorials & on tech forums. It's getting to the point to where I need one, and have plenty of spare components to build one, probably two. Once setup, as I understand, the maintenance is low, and one doesn't have to have a super powerful computer for this, my plans are to repurpose an AMD Athlon II x4 630 2.8GHz quad core CPU, one of AMD's first true quad model lineup that caught Intel with their pants down , to build one. Just unsure whether a SFF desktop computer is large enough (could be if two 2.5" HDD's are used in a RAID setup), or would need a tower for extra drives (have both types). Since it won't be used to browse the Web, no need for a SSD, extra GPU, just run whatever graphics ships with the MB. However, reliable HDD's are a must, one reason why I love the WD RE4 line (the older SATA-2 models). These are basically upgraded Caviar Blacks, with twice the cache & same 5 year warranty, in fact have a 1TiB one coming in next week, under warranty until mid-2018 (found on eBay for $32 w/free shipping).
A RAID setup using two identical HDD's may be a good idea, in case one fails, the system will run on the other until replaced. No one wants their Server to totally go down, that defeats the purpose of having one. I also recommend to connect to a properly sized UPS, so that it can be safely powered down, and also protects against momentary outages, surges, good models will provide clean & stable power to extend the life of the hardware.
Just as the Linux community is very diverse, the same can be stated about those running Server clients. No 'one size fits all' applies here.