Depends alot on what you are going to do with your ipad... but my favorite apps so far:
Paper (P53 Pencil)
Tapatalk (paid)
Opera browser
Youtube app
Google apps (mail, drive, maps, earth)
Living planet (photo app)
PS Touch (unfortunately it is now removed from appstore)
Pinnacle Studio (just for fun)
Swype (3rd party keyboard if you like to swype)
Procam xl 2
iThoughts
WikiLinks
Noteshelf
Artrage
Procreate
Pixelmator
Skype
Hangouts
SkySafari 4(?)
DistantSun(max)
Documents