I visited all three and it's interesting how different they are.

My ranking:

Obama - far and away number one
Hillary - second best
McCain - very distant third

http://www.barackobama.com/index.php
http://www.hillaryclinton.com/
http://www.johnmccain.com/pathtovictory/democrats.htm?s=google&t=electability

I agree with those rankings.

The Obama people are doing a great job producing his online efforts.

Clinton's site is decent, and at times very good.

IMHO, the production values directly relate to the appeal and weighting the campaigns give to younger voters.