Without a doubt the G1... The Android operating system is the hot and coming OS, and will be gaining huge amounts of market share in the next year. It is expected to surpass both RIM/Blackberry and the Iphone by 2012, although I personally suspect it will happen before that. Right now there are over 20,000 applications available for download for Android, and the number is growing at 3000 to 4000 per month. About 2/3 of these apps are free.
If you want to check out what's available go to http://androlib.com
On that note, the G1 was the very first Android phone released, and is over a year old now. T-Mobile has several other Android phones available, the My Touch 3G, Motorola Cliq, and 1 or 2 others. I personally would recommend picking one of these newer and faster Android phones rather than the G1.