In my opinion, Alaska Airlines has the best program for you, as long as you would only be flying enough to make low level elite on any of the major carriers. I say that because as a low level elite, upgrades are going to be rare, at higher levels they become much more common.

AS will let you earn and burn miles on a ton of carriers on varying alliances. As your goal is to get free tickets, I think this setup would be most ideal (yet not the ONLY solution) for you.