Your ISP probably has specific ones they recommend using, those should be available on their support site. However at times these do not function properly with all devices. I have had luck using open DNS servers (which are free to use) when using an older router and setting my Xbox in a DMZ, they are:
208.67.222.222
208.67.220.220
Of course when using these DNS servers you still need your assigned IP, Subnet and Gateway addresses assigned by your router/ISP.