Aim for low power consumption in an HTPC since that means less heat, less money to run it, and most importantly fewer fans, moving parts, and noise necessary to cool it.
Look at the AMD X2 3800 EE. It runs at just 35 W. In some cases you can run passive cooling without a problem. 780G mobos like the one above perform excellent hardware decoding of popular video codecs which means that the CPU is relatively unloaded, even when playing 1080p H.264-encoded content.