The CC2531; there's a thread here: Zigbee Packet Sniffer-Cheap &Works
The free software (download link is in the forum) isn't the easiest to use but the more I've worked with it the more I appreciate how much you can do with it (looking at different Zigbee channels, I discovered two previously unknown Zigbee networks in my house, one of which was coming from my Echo Show 2 and the other I still haven't pinned down). It helps to know what you are looking at, as well, especially when setting up the filters. It has some nice features for templating them, and they can be saved for re-use.
