Sidekick II (or as people who are really into it know it as, a hiptop). I'm addicted, it's so sad. I have AIM/Web on it so I'm always on (plus tetris in class haha). It's gotten to the point where people are like "Mireille! You have LIVE company. Stop chatting.)
I wouldn't recommend getting one, just yet though. There are plans for a Sidekick III with bluetooth, highspeed internet and MP3ness...Which I think might be my next years christmas gift.
Tmobile also is a little shoddy at times (um, paris hilton hack, anybody?) but when it is working, I have more service in doors than with the other sprint phones I've had.