MapQuest and Google Maps
I use both, sometimes to find directions to the same place. I can't speak for Blighty, but in California, MapQuest is more accurate. Google's overhead and street images are more up-to-date, tis true, but that doesn't mean much if the basic function doesn't work.
Why doesn't Google work sometimes? Because it fails to complain sometimes if the input is bad. Mapquest always normalizes (USA based, at least) addresses to the postal service, making sure the city and postal code actually match. Feed Google GI and you get GO: it's all understanding, you see. Feed Mapquest GI and it gives a list of specific choices asking what you meant.
How ironic. I just realized that Google Maps is a directions service that doesn't ask for directions!