I'm not aware of much in the way of statistics to prove one airline is better than another in terms of "saver" award availability. However, I would concur that American's AAdvantage program is at the top of the pack and Continental's NonePass is at the bottom.

If you review threads across FlyerTalk on award availability, it seems clear that most people who participate in multiple programs would also put AA at the top. Individually, this is anecdotal information, but taken together I believe it is a compelling endorsement of AAdvantage.