I use Wi-Spy
http://www.metageek.net/
I used to use "/interface wireless scan", but that only indicates the number of aps on that frequency, not how busy each is. One busy ap on your frequency can be worse than 6 slow aps.
ADD: If you purchase one of these, DON'T CHEAP OUT! The $99 unit (2.4i) does not have an external antenna. The $199 unit (2.4x) has an external antenna connector, so with a pigtail you can connect it to the antenna you are checking. It is worth it!