Please steer me to list of approved cable modems

Cox Compatible Cable Modems and Gateways

Between those three, I wouldn't bother with the 6121. It's basically the same modem as the 6141 but with half as many downstream channels.

The 6141 has admittedly had some growing pains on our system, but the kinks seem to be worked out as of the latest firmware, and I've never had trouble with the one I got last October. The main advantage it has is a built-in MoCA filter, simplifying installation of our Trio/Whole Home DVR service.
The 6180 is based on a different chipset from the 6141, essentially the same hardware platform as the Cisco DPC3010 but built and programmed by Motorola, and some people have a brand preference.

Aside from the MoCA filter, the 6141 and 6180 should be functionally identical as far as we're concerned.