hmmmmmm complex question that one. Because modems and NAT routers are typically tied together it gets a bit messy.

I have for sometime support (and still do) the simple but basic TpLink TD-8840, this is a Broadcom based modem that works well with the Telecom Broadcom based line cards and the newer Ikanos based ADSL/ADSL2+/VDSL2 based line cards. they are not expensive and have more than enough resources to hand to deal with quite large connection numbers that you would encounter in a 20Mb/s ADSL2+ connection. They seem to have no worries with large NAT loading (ie lots of torrent connections) and the TP -Link firmware seems to deal with cleaning up after itself well enough not to need regular rebooting.

So if it were me, tomorrow wanting to install the best modem for a typical Small business or home, then the TP-Link, or Dynalink wins. If you want bigger loading, then you need to look at solutions that provide separate modem and NAT functions. Fortunately newer EUBA and VDSL2 deliveries are using PPPoE, this opens the access to a wider range or modems and ready interface to whatever NAT router you feel you need.

This conversation should continue, there are lots of options and many ways to kill the cat which I would like to explore, the very near advent of VDSL2 and EUBA services (plus FTTH) are making this space very interesting indeed.

Cyril