That is a bit of a loaded question. While there are going to be forum members that have a router model that has worked well for them, that doesn't mean that it would be the model that will necessarily work well in your situation.
A lot has to do with what models are available in your location, how much you are willing to spend, and how many additional security, and other features (VPN?) that you desire.
You mentioned a modem router (combination?), so I have assume you are talking ADSL service. That criteria is going to further limit your choices. Myself, I'm not a big fan of combination units as you have to replace the entire thing if one part fails. They also tend to have fewer features as they are , for the most part, for the consumer market and not enterprise.
I would say, that with a few exceptions, the majority of modern routers will work well with 3CX. Some may require some tweaking to get them to "play nice" with SIP.
The router you choose has to support port forwarding, which almost all are able to handle, the rest is "icing".
Personally, I use a number of D-Link DIR-615's running DD-WRT firmware, and have not had an issue. I have tried a number of "basic" routers, and not found one that has given me problems. I just prefer the additional options that DD-WRT offers.
I'm sure you will get some responses from other forum members regarding their preferences.