I've never heard of buying air popped popcorn, just the poppers and popping your own! (I've never typed the word "pop" so many times in one sentence!
)
Anyway, if you are doing the Core version of WW, and really even if you aren't, the best kind to use is the microwave 94%FF. Act II makes one, as does Orville Redenbacher and Pop Secret too, I think. It's really good and they have the mini bags, which are low in points and a good portion size.