Best ones eh?
Goodreader - I have a lot of game PDFs and while iBooks can read them, Goodreader is better for organizing and reading.
Kindle - I don't have a great deal of eBooks but the ones I have, I read through the Kindle app.
Politico - I don't necessarily keep up on politics 24x7 but during elections it is handy to have a dedicated app to follow what's going on.
For games:
Carmageddon - Drive around running over pedestrians. Can't ask for a better, time wasting game

Unfortunately they did an update which killed it on the 1st Gen iPad. Still works on my iPhone though.
Duke 3D - The original Duke Nukem 3D on the iPad. Great fun.
Ticket to Ride - Board game transferred to the iPad. And you can play with others either by handing it to the other person on their turn or even two devices.
I have had fun with GWars: Touch and RealRacing HD although I don't play them as much as I play Carmageddon.
I have several music related apps but have only really used iElectribe with any regularity. I use the presets as drum machines when I'm practicing guitar.
I've used Evernote with regularity and it works reasonably well. One unfortunate issue is if I'm using the iPad without WiFi, the stuff I've written isn't always on the iPad. So I suspect they store the data in "The Cloud", maybe after a certain period of inactivity.
The others I use are the web browser and email. I play tunes through my stereo or here at work through my PC speakers.
Carl