How can I use my Mac to determine the optimal WiFi channel to use with my wireless router? I'm setting up a new router and want to manually select the best channel, how can I run a scan of the networks around me to figure out which channel will have the best performance?
You can easily determine the best channels to use in the native Wireless Diagnostics scanner:


Open the "Wireless Diagnostics" Application
Ignore the prompts and open "Window", "Scan" in the menu bar
Press "Scan Now"


The recommended channel(s) for both 2.4 GHz and 5 GHz will now be present in the Summary pane.



More info on OSXDaily.