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.