It took me a while to have my decision made and here it is:
Note taker HD - Handwritten Notes
Mercury Pro - Web browser
GoodReader - File viewer and manager
iBooks, FreeBooks - Viewer for books reading
GoSkyWatchP - Starwalking application
Smurfs - game
PlayPianoPlus - piano application
That's what I keep open mostly all the time and I have fun because they work great. This doesn't mean that I wouldn't like to have improvements and other features to make them even better but however they are already so good that I could stay with them also as they are. Of course this is very subjective. Give them a try and tell us what you think.
If I should choose just one of them I would say that Note taker HD is the one that really makes the difference and transformed my iPad in something really powerful completly different of any other computer that I had. The second is iBooks or FreeBooks. All my library stays with me now.