A good cable modem?

You need to ask Comcast for a list of their approved modems. I had to do that; after I looked at their list, I did some research and then selected one. We do not work for Comcast, so it is best for you to do the research:

https://www.xfinity.com/support/articles/list-of-approved-cable-modems

https://approvedmodemlist.com/comcast-xfinity-approved-modems/

Personally, I have had very good luck with Arris modems with Comcast.

You need one that is compatible with the Comcast service. Any compatible brand name modem will work. You'll have to register it with Comcast in order to get it working on their service.