This is a tricky question. The simple answer is a phone with the lowest frequency, such as direct 6.0 which operates in the 1.9 GHz range. Anything lower can be found, but will most likely be inference with microwaves or wireless networks. Flat out though, the EnGenius cordless phone is the longest range cordless phone that is legal in the US.
This is tricky though because what really matters is the amount of objects between the handset and the base. Regardless of the operating frequency of the phone, better range will ultimately be achieved by moving the based to a place in the house with the least amount of object between it and the handset. So before you go looking for a new phone, try moving the base to closer to the outside and see if your range improves.