Personally, I think SPG (Starwood Preferred Guest, Sheraton, Westin, W, St. Regis, Four Points) is the best, I have used up tons of free hotel rooms already! And I still have another 25 freebies (or less, 12-15 at the W or St. Regis) to use up! You get the points from hotel stays and especialy from using the MBNA SPG credit card. (Then when you get gold ot platinum, they throw even more points at you)
Aeroplan and Airmiles are crap... but I still have to use up all that too...