It changes over time.
Default
1. iPod
2. Notes
3. YouTube
4. Mail
5. Safari - This is usually my #1, but nomore WiFi in my house and have a 200 MB limit
Third party
1. Opera Mini - Save data
2. MobileRSS Free
3. iData Usage
4. ESPN Scorecenter
5. MiTube (aka MxTube in Cydia)
Shopkick was my absolute fav app until they banned me and they got stingy with the kickbacks to the point that it wasn't worth it anymore. I got over $100 of giftcards from them in a span of one month before I got banned and they got really strict.
Lately, I haven't much of an app freak or even play mobile games all that much. I get bored playing Angry Birds real fast and already beaten some of the ones I have. Just give me an internet browser, a good multimedia player, text, notes, and I'm content. A private storage app and VLC would be nice too. Probably why I may shift to WP7 or BlackBerry OS 7 for my next phone. Don't want to be stuck on iOS for another two years to make it a total of six years. I currently have 75 apps from the App Store and many (about 85%) aren't a necessity to me. Including the default ones, there is only 20-25 I would need or constantly use. The browser could solve most of my questions.