My Wi-Fi base station has an automatic channel feature that works fairly well by sampling each channel for interference and determining which one theoretically will work best, but is based on the location of the base station and not the devices attempting to receive it. I did some extended analysis of it recently (manually changing channels among other things) as a result of trying to diagnose a DSL modem problem that was disguising itself as a Wi-Fi router issue. Ultimately it has me on Channel 11 for 2.4 gHz even though another network in my area uses Channel 11. Wi-Fi channels have significant overlap so it is necessary to move at least 5 channels away from one being used to completely avoid its interference. For example, to completely avoid interference from Channel 11 it would be necessary to move to Channel 6 or below. At certain times of day, if I reset my automatic setting, it will select Channel 1 instead of 11, but there are typically 3 or 4 networks in my area using Channel 1 at certain times of day. Right now I have neighbors on channels 1, 3, 6, 10 and 11, so it is impossible to completely avoid the interference from all of them. It's also a matter of how strong the signals from competing Wi-Fi networks are at your receiving location, not just the number of networks using a particular channel.
I'm sorry there is no easy answer for this, and I can't just tell you a channel. A reception analyzer as hernanu suggests would be useful for this if it shows an actual analysis of the signal strengths across the entire 802.11 spectrum, so you can see which channel would overlap the frequencies with the least interference. If your base station has an automatic setting it is probably best and easiest to use it, but it may not be fool-proof. A simple default will probably put you on Channel 6, which usually puts you in competition with everyone else in your neighborhood who leaves their Wi-Fi on the default channel of 6.