Google just gave us a new one. And boy is it good. First, it gives us what Google does best: a simple search. You can simply type the whole search in the single simple input field and get your results. Second, it gives a very slick interface that allows us to interact with the map without getting a full page refresh (I assume it's based on the same concepts as their Gmail interface). The last feature, and this is under-appreciated one, is that the maps they give us are big and clear. A search for a particular address will give you a zoom level where streets have width (rather than lines). The maps are the easiest to read of any of the map services I've used.
Yahoo! Maps, Mapquest, and Microsoft MapPoint: they aren't even in the same ballpark.