What motherboard should I choose for a Ryzen 9 9950X3D build?

A strong B850 option is the MSI B850 Tomahawk MAX, or the B850M Mortar if you need micro-ATX. It has the features most people actually use without the extra compromises that can come with higher-end chipsets. The Gigabyte B850 Aorus Elite is another good higher-end choice. Check the manual carefully for M.2 lane sharing, since some boards reduce GPU bandwidth when certain secondary M.2 slots are populated.

You probably don't need an ultra-expensive board for those requirements. Most B850 boards support 64 GB or more of DDR5, include multiple M.2 slots, and provide a full-speed PCIe slot for a current high-end GPU. Reliability is difficult to predict from brand alone; look for detailed reviews, solid VRM cooling, and a board with the rear clear-CMOS button you specifically need. For a wireless controller, a dedicated 2.4 GHz USB receiver usually gives lower and more consistent latency than Bluetooth.

X870 mainly adds features such as USB4 rather than automatically improving reliability or gaming performance. On some X870 boards, USB4 and the secondary M.2 slot share bandwidth, and other models may reduce the graphics slot from x16 to x8 when additional storage is installed. If you want two or three fast M.2 drives without sacrificing GPU lanes, B850 can actually be the cleaner option. Whichever chipset you choose, update the BIOS before putting the 9950X3D under load, and confirm that the exact model has a rear clear-CMOS button.