I am a fan of inSSIDer, however, it doesn't give you the entire answer. It tells you what channels are in use by various SSIDs, but not how much any of theses SSIDs are using the Internet. Better to share a channel with five old ladies that just check their e-mail occasionally than with with a single user that is constantly streaming videos.
Use inSSIDer as a starting point and then experiment to see what channel works best for you.
As previously mentioned AUTO channel selection doesn't work that well in picking the "best channel" and certain devices have problems connecting when the channel changes.