Best PC motherboards for USB Audio?

The best sounding motherboard is no motherboard. PC audio is a challenge, and unless you want to consider a lot of pricey stuff to compensate, you are better off investing in a dedicated streamer/network player.
Not to retype same stuff for 100th time, but in essence you want to eliminate processes running, minimize OS(think Linux)go to small linear power supply and separate your power and signal where you can. Keeping mechanical drives in another room and connect ideally by ethernet will also be a big improvement. computeraudiophile site is expert on much of the related minutia I mention, although plenty of related comments here. Cheers,
Spencer

@wqgq_641,
If you have to stick to one computer for general use and audio use, then @erik_squires suggestions regarding power supply and isolation are on point. 

However, consider if you can instead sticking with the current pc for other use and invest in a dedicated network player running linux. Moving mechanical spinning discs to a NAS in another room is absolutely a benefit in sound quality, regardless of the USB/ethernet discussion. If your library is small external SSDs might be another option(although generally too expensive IMHO).

Have you read about projects utilizing Raspberry Pi with Volumio app? They sound startling good vs. most of the PC based setups I've heard and they are super affordable. With your building skills, could be a great fit! I have a friend who's built a few that I could hook you up with for any email questions etc., if you like that idea. DIYaudio has lots of coverage on them. Cheers,
Spencer