Goaltending is the most difficult position to predict and the volatile in terms of long term performance. Ranking young goalies as far as their careers go is almost impossible; you may as well throw darts at a board IMO. Factors include what teams they end up playing for, what kind of competition they have, injuries, and of course opportunities. Also, how willing are they to take a pay cut in the salary cap era? Niemi wasn't. Now he's sitting on the sidelines and probably won't be resigned by an NHL team.
If you are looking at pure upside while ignoring all these factors, then I would say the elite goaltending prospects would include:
Bernier
Markstrom
Varlamov
Neuvirth
Campbell
These are guys that could POTENTIALLY have a high-end NHL career if the stars align and they stay healthy on a competitive team. Unlikely any would reach the lofty heights that Roy or Brodeur have reached. These guys are rare exceptions.
Next level down I would probably place:
Pavelec
Lehner
Schneider
Salak
The other guys I am not familiar enough with to rank them. Hopefully Justin Goldman stops by for a comment.
Good luck!