I'd have to say my Nokia 3650.
It functions as my:
-Cell phone (obviously
)
-Camera (actually takes surprisingly good pictures for the web)
-PDA (synchronizes with Outlook via infrared/Bluetooth; pops up reminder alarms based on calendar events)
-Portable game device (it even runs a full color version of Doom, and of course, the classic Tetris)
-Alarm clock (a must-have in university
)
It's a Series 60 phone running Symbian OS, so there are loads of Java applications that can be downloaded to increase the functionality of the phone. There's one little app that allows you to use the IR transmitter of the phone as a universal remote control for your TV/VCR. Even the Opera browser has a Symbian OS version.
The replaceable multimedia memory card (MMC) is a big plus, since you can effectively upgrade the memory of the phone to the size of the largest MMC available (Is it 512MB? Not sure...). More storage space for pictures, videos, ringtones etc.
And lastly (but a very important feature), it's a tri-band world phone capable of functioning with GSM 900/1800/1900 networks. So I can take it with me when I'm out of the country on vacation.