TechCrunch has done a fairly detailed review round-up of the major online mapping services, including Ask, Google, MapQuest, Windows Live, and Yahoo!
It's worth a read, but if you're too lazy, here's the bottom line: “Overall, Yahoo Maps was by far the best application tested.” Ok - sounds good to me. I'll stick with Yahoo Maps for now.
Yeah, I'm a committed Yahoo Maps guy, too. The dynamic, multi-point driving directions and the search integration is awesome.