The Nexus One has been rooted, so I think you'd be good going with that option.
The Moto Droid is another option. More people use the Droid, so there is a lot more information (so to speak) available with that device.
The ADP2 (Google Ion) is basically the MyTouch 3G and it is becoming a little dated. I'd hope that Google releases an ADP3 before long as phone specs (and OS's) are moving into the future.