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While I don't have an iPhone, here are my 5 most used functions:

1. Messaging
2. Calendar
3. ToDo List
4. Games
5. Internet
 
1. Email
2. music (Airplay/Spotify/Rdio/iTunes music)
3. Downcast
4. Remote control (Roomie/Rowmote Pro)
5. Twitter
 
1. Social apps(messaging/Facebook/etc)

2. Camera(+light editing)

3. Music(spotify)

4. Shopping apps

5. Email


I can honestly say I use my phone more than my regular computers these days.
 
  1. Messages
  2. Phone
  3. Camera
  4. Mail
  5. Safari

Prior to getting a Kindle and iPad, Stanza (epub reader) and Safari were numbers 1 and 2 respectively.
 
1. Fondling it
2. Smelling the leather case
3. Telling my friend I have one and he doesn't
4. Calling
5. Email
 
1. iPod (SmartPlayer)
2. Messaging (stock app)
3. Mail (stock app)
4. Surfing (stock app)
5. Tie between calls and camera? (stock apps)
 
1. Email/Text
2. Information/Reference (Browser, Siri, Calculator)
3. Photo/Video
4. Entertainment
5. Scheduling (Reminders/Timers/Alarms)
5.5 Flashlight
 
According to battery usage in settings, Safari, messages, music, mail, Instagram
 
In no particular order as I am always using one of the following:
  • Music
  • Facebook
  • Email
  • Messages
  • Safari
  • Camera (ok, that's six. I'm cheating)
Also used very frequently (often several times a day):
  • Maps
  • YouTube (especially as a Red subscriber)
  • Calendar
  • Notes
  • Timer (especially via Siri: "set a timer for 5 minutes")
  • Stocks (and various banking apps for my different financial accounts)
  • Weather
  • Flashlight
  • TV Remote
  • Slack
  • Evernote (and companion apps, eg, Scannable)
  • Various fitness related apps (eg. iSmoothRun)
There are also things like Passbook, some travel related apps (eg. United), Fandango, some games, etc, that I don't use every day, but I use them frequently enough to have them be an important part of my mobile experience.
 
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