There are so many it's hard to mention just 3... so I'll cheat and give you 3 lists:
The 3 I use the most for reading:
1.
iBooks - warts and all, it's the easiest for my many non-DRM epubs
2.
Zinio - I have 3 magazine subscriptions through them. They have worked very hard to address the initial release concerns. The current version (1.5) is superb.
3.
Goodreader - if it only read archived PDFs it'd be perfect instead of only nearly so.
The 3 non-readers I use the most:
1.
IMDB - very nicely formatted for the iPad. If you watch movies or TV this is all you need.
2.
Airvideo - quick and easy video converter that recognizes all formats and transcodes and streams (or converts for syncing through iTunes) even HD mkvs perfectly. It will even streams from your home PC through the internet in a pinch although I wouldn't do that through 3G on a limited plan
3.
BBC News - very well implemented news-highlights from around the world plus BBC radio feed streamer.
My 3 favorite time wasters:
1.
Let's Golf - cartoonish but very playable golf sim.
2.
Koi Pond HD - very customizable fish pond simulator.
3.
Wesnoth - Turn-based fantasy strategy game.