This is a tough comparision since Bouwmeester has played (and played well) at the NHL level while Phaneuf and Barker are still in juniors and Johnson hasn't even played a year of college hockey. Right now it would be:
1) Bouwmeester
2) Phaneuf
3) Barker
4) Johnson
In the future, tough to say, although IMO, Bouwmeester will be another Niedermayer, a player who is good for many years before (potentially) getting recognized as the best at his position. I think Phaneuf is a much more complete prospect than Barker, who I'm not a big fan of. Still, they both deserve an objective eye once they hit the pros. As for Johnson, I'm a HUGE fan of his, but until he plays NCAA hockey next year, I lack an additional frame of reference for his developmental path. That being said, he combines many of the best attributes of both Barker and Phaneuf, and has the potential to be better than either.