Well my situation is interesting. I'm based out of ICN so my two main airlines are Asiana and Korean Air. They both have very good expiration times at their top tiers, 12 years and non-expiring, respectively. However, to get to top tier on both airlines requires 1 million miles which is a bit ludicrous. To get lounge access quicker I've been putting my non-Korean air and non-Asiana flights on Lufthansa (Star Alliance) and Delta (Sky Team), which leaves me with just One World to complete. I first put my miles on Mexicana since I was flying to Mexico frequently but that turned out to be a bad move. Now I have them in Finnair because I was flying out of Helsinki a good deal as well but realized a bit too late that Finnair and Marco Polo both have pretty terrible expiration times on their miles. My travel patterns are very erratic and I never really know where I am going so I'm just covering all my continental bases with my programs.
I might look into AA, but I'd prefer a program where if I don't use my miles in the course of a few years they won't vanish.