Satguide, Map My India. I have heard latter is better.
My 2 cents.
I have been using Windows mobile for the last 4 years or so and have used GPS often.
quick GPS really works wonders. I would say it gets a fix in 5 seconds and not even 20-30 secs as someone wrote.
As far as maps are concerned, I have used mapking, Satguide, MMI and google maps.
Mapking, Satguide and MMI of course are offline and the whole map resides on the device so you do not need data to access. The applications of course are all paid. GPS of course being free. Some of the Asus phones come bundled with satguide. Among these 3 definitely MMI is the best.
Google mpas advantages
1. Free.
2. Satellite view - not there in any of the others.
3. Integrates with other google services like lattitude and Buzz.
4. The new layers feature is great.
5. Voice navigation already available for Android and BB. Voice search feature present for Windows mobile.
6. AGPS - which gets your location based on the relation to mobile towers. Very useful for devices which even do not have GPS.
Disadvantages
1. Data costs
2. Battery drains faster since you are using GPS and data as opposed to other softwares where you are going to use only GPS.
3. Areas without network, no use.