Depends on what you speeds your wanting as well as who your ISP is.
The modem needs to be compatible with your ISP.
I'm a fan of the CM1000 and CM1100 modems as their docsis 3.1 gigabit capable and the cm1100 also supports wan aggregation.
but again, speed needed and compatbility is important. Whats the point of buying a gigabit modem if you're only needing 100mbps service.
