Google's Maps application is simply the best out there. It's only going to get better with monthly updates that will add features and perfect the interface. Bing Maps on Windows Mobile is surprisingly good but it doesn't have the same layers and granular features as Google Maps.
The iPhone's Maps application has barely changed since 2007. Good alternatives in the app Store can be expensive and take up as much as 1-2 Gigs of precious phone storage. Google's app, on the other hand, takes up almost no space on the phone because everything is in the cloud.
The only downside is that Google's Maps uses more data and re-routing when outside of a data area. If you require offline maps, there are some in the Android Market. Also, HTC is offering a free version of offline Maps on their phones.