As NM said, there are a lot of different airlines, thus you could join each carriers' FF program, however this would dilute your future benefits. However, some of the airlines that you have flown on are part of global alliances, so you could credit flights from a number of airlines to a one FF program (thus maximising any FF benefits). For example, using the airlines you have flown, they are part of the following alliances:
Oneworld - Qantas, American,
Star Alliance - Thai, United
Skyteam - NWA
I'm not aware that Air Asia are part of an alliance, being a low cost carrier. You also mention Air Alaska - do you mean Alaskan Airlines (with flight numbers starting with AS)??
So you need to look at which alliances you did the most flying, as well as your proposed flying in the future, to try and ascertain which specific airline frequent flyer programs you would be able to maximise benefits. It would seem that of the flight you mention, the mileage appears to be split fairly evenly between Star Alliance and Oneworld.
I would presume that your flying on NWA, Mexicana, United and American were more likely to be within Continental USA, Canada or South America,. so shorter flights. It would be beneficial to know all the flights you took (airline and city/city) to be able to further crystalise my thoughts.
Now with a couple of the airlines, they appear to be flexible as to where you credit the flight mileage (Mexicana and [presumably] Alaskan Airlines).
Given you have stated expected travel to America, do you have any feel as to which airline you would likely book a ticket (as it may depend on whether this is personal or business travel).
Personally, based on the presumptions above, I would be leaning towards crediting flights to either the American Airlines or Qantas FF programs (for those flight on QF, AA, AS and possibly Mexicana) and Thai (for United and Thai).
I can go into more depth as to why...but let's jump the first hurdle and see what you think...
