Just got a R7 5800X, having a tough choice on motherboard selection. They all look awesome.
B550 Chipset.
I've had good success with my Asus TUF Sabertooth R2.0 with the 8350, and an AsRock 970 Extreme 4 with a X2 555BE
3 boards in the Running.
AsRock B550 Steel Legend, Extreme 4.
Asus ROG Strix B550-A Gaming
Asus did not bios update the TUF B550 Gaming Pro to support the 5800X unfortunately...
Differences:
Steel Legend and Extreme 4 have Debug Code LED, support 4733+ Ram OC
Strix has Bios Flashback, Supports Ram OC 5100.
Steel Legend and Strix have DP and HDMI
Extreme 4 has Power and Reset Buttons.
I understand a APU must be used to utilize the IGPs.
All are 14 Phases and Support 5800X.
AsRock seems to be on the ball with Bios Updates.
I will be using 2x16GB Ram.
This rig is not for me but my lady.