After more testing, research and testing, I think the 3 distros I'm looking at using for my future HTPC build are these 3. Can any of you more knowledgeable folks weigh in with your opinions on which might be the better one to go with?

My build will be a 240 gig SSD boot drive and 6, 4TB data drives that I'll assemble into a software RAID 6 array via mdadm (using webmin as a GUI into mdadm). Media will be served via Plex and Samba so programs like Kodi can access the share directly.

I want ROCK SOLID STABILITY first and foremost. Security updates are important as well but this won't be sitting on the internet (will be behind a consumer router) and the ONLY program this will be running is Plex. I'm a n00b with Linux but have reading comprehension skills can can follow wikis. I don't want to get into a situation where some update kills my boot loader and I'm stuck looking at a screen with a boot error that I can't decipher and the RAID array is down and my wife and kids keep screaming every 5 minutes that they can't watch anything.

They're all rock-solid as far as stability is concerned. I'd lean more toward Mint or Lubuntu for ease of use. If you really are a neophyte, I think you'll find Debian a bit daunting.

What does Plex recommend?

Plex doesn't have an an official recommendation.

Make it headless. Don't bother installing something that needs a monitor / mouse / keyboard. After the initial installation stuff it in a closet or under the bed and access it via ssh. Don't bother with gui if you don't need to. Just adds overhead. The command line is quite easy once you grasp a few basic concepts.

Of those 3 I would pick Debian. I went with Ubuntu Server for my htpc / server only because Debian lacks the mythtv packages I need, and I have no trust in deb-multimedia.

Debian.
It's Universal.

I'd use the LTS sort of versions. Mint offers a lot of advantages for workstation users as it adds in a lot of stuff for them. You want more of a server based install. I'd consider Ubuntu LTS over mint. The less bling the better. Consider even going to Suse Enterprise from SuseStudio.com if you want enterprise level. Even Centos may be a consideration.

Rock solid is still a function of your hardware and the distro. Server based kernels may have more conservative settings which gets back to my not liking mint for this use. I'd see if a server distro would be usable.