See this link:
http://www.frequentflier.com/blog/an...ility-goes-to/
Per this survey: Air Canada/Aeroplan is in the middle of the pack (11/20). Of the 4 US legacy carriers, UA offers more availability, but American Airlines, Delta, and US Airways offer less availability.
However, the availability is shown to have gone down significantly for a given period of time this year compared to same period last year.