Hands down the best. Provides the most information and best range of self-service functionality... all of which really can be a time-saver.

If I may offer a few suggestsions, here are some things I'd like to see:
- Ability to waitlist for airport standby without going to a kiosk or agent (perhaps using GPS/geolocating)
- Boarding notifications
- Real-time bag tracing (like DL)
- Expert Mode fare class availability built into the flight status tool
- Ability to initiate flight changes within reservation page (outside of checkin window)
- Ability to accept schedule changes within app (instead of dead end with 'waitlisted' notifications)

Some of the foregoing are easy to do on United.com or at a kiosk, but clicking through to the website, toggling the full site, resizing screen, etc. on a phone is a little frustrating. All that said, while it's not a killer app, it is a damn good one!