Debian jessie. As a default OS, you need to have a very smooth, easy to use OS. Debian does this quite well, even more so as it turns stable.
While I like Arch, currently use it and would probable use it on the Pyra too, it isn't suitable as a "default OS". While it is quite stable, even with the always current packages, it /can/ crap out on you with varius things.. This is not something that is wanted for a default os.
As an added bonus for n00bs to linux:
Say you want to find a cool music player. People would get into their web browser of choice and search for linux music player. What are the chances they they will find the pacman command? They are almost assured to find an apt-get command. Due to the insane popularity of Ubuntu, a huge portion of the "linux" help pages are really Ubuntu help pages. Luckly, for the most part, this also means Debian. It doesn't mean Arch though, or OpenSuse, or any none Debian based. Granted, this wouldn't effect most of you here, but it should be taken into account.