The big AM in-band channels are as follows:
Channel 6 - 27025 - The Super Bowl - the big one, some of these guys are splattering so much junk it gets all over the band
Channel 11 - 27085 - the original calling channel from the 23 channel CB days
Channel 15 - 27135 - seems to be the alternate to channel 11 in some areas
Channel 26 - 27265 - Often very busy in the southern USA
Channel 28 - 27285 - Similar to ch 26 / 27.265 MHz - lots of high power stations heard here during band openings
If all these channels are busy, check the English language low band calling channel, 26915 AM, and nearby frequencies, namely 26905, 26885, 26835, 26815 and 26775 seem to be the most popular ones. I've heard stations in Texas talking to each other on 26955 several times, some of them with broadcast studio quality audio...
Another one worth mentioning is channel 9, 27065 AM. It is used as a high-power calling frequency for Spanish speaking operators. When you hear Spanish language chatter on 27065, check the out-of-band calling frequencies for activity coming out of Latin America, including the following:
26555 LSB - Spanish language, Mexico, Dominican Republic, Puerto Rico - low band SSB calling freq
26565 AM - Spanish language, Mexico
26585 AM - Mexican truckers calling frequency
26705 AM - Puerto Rico calling frequency (I've heard Miami, FL working stations in PR here before)
26715 AM - Puerto Rico calling frequency (alternate to 26.705 MHz)
26725 AM - alternate to 26.705 MHz and 26.715 MHz
27455 USB - high band SSB calling freq (note that 26555 is channel 4 down one band, 27455 is channel 4 up one band)
27665 to 27705 USB/LSB in 5 kHz steps, generally centered around 27665 USB and 27695 USB, lots of Spanish activity