I'm a big fan of CentOS. It's a little trickier to set up but is a clone of Red Hat Linux which is pretty much the gold standard for open source servers. Tried a number of NAS packages, including FreeNAS, and several Linux server distributions but found CentOS to be the most reliable and stable. The main reason for not using FreeNAS was the file system which makes it incompatible with Linux.
Originally I was using a retired AMD PC with an assortment of drives and SATA cards. It worked surprisingly well considering the menagerie of hardware that was not meant for server or NAS use. It's a great way to build a NAS on the cheap.
Switched to a SuperMicro mini-ITX motherboard with an Atom D525 processor and 4GB memory in a tiny mini-ITX case made for NAS. Was using 2TB WD Green drives but they had about a 50% failure rate. When WD Red drives became available I replaced any failing drive with those. A few OS updates later it eventually got so it wouldn't run CentOS or FreeNAS well.
Another project was an mATX SuperMicro motherboard and Core I3 processor with 16GB of RAM in a full ATX case that has 9x5-1/4 drive bays in front. Slid in three 3x5-1/4 to 4x3-1/2 hot swap adapters so it has 12 hot swap drive bays in front.
The latest iteration is an ASRock server motherboard with an Intel Core I3 and 16GB of RAM in a mini-ITX case with 5 hot swap bays. Picked up some refurbished HGST server drives on the cheap to round it out.