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There are lots of motherboards to choose from that fit the bill, but I've gone for the Intel DH61AG. This board supports LGA1155 Core i3, i5, and i7 processors up to 65W TDP, two RAM slots supporting 16GB of RAM, gigabit LAN support, ten USB 2.0 ports, two USB 3.0, and a pair of SATA ports.
Everything a good all-in-one PC needs.
Image source: Intel.