i must echo RedRallyeZ's question. Most ISPs either require you to use their modem ,or if you want to install your own, only allow a specified list-at least that is what my ISP, Comast, does. Most ISPs won't help or allow you If you just install a random modem, as they cannot verify that it doesn't have issues with their network.
So the better question should be what is the best approved modem for your ISP?