I would personally recommend the Project Sputnik hardware from Dell http://www.dell.com/learn/us/en/555/campaigns/xps-linux-laptop?c=us&l=en&s=biz, in particular the XPS 13 Developer Edition .They put a decent amount of effort into making sure the BIOS and firmware at least work with the latest version of Ubuntu and mainline these for the most part. Worst comes to worst, you can just repackage the stuff from Ubuntu but I'd say pretty much everything is working as well as it should on mainline Arch today.
Not exactly open-source/FSF-endorsed but https://system76.com/ sells laptops with Ubuntu-preinstalled so those are also guaranteed to run Arch.
we should propably collect such knowledge in the wiki. something like "hardware guide" or similar?