Well if you have the Orbx TX PNW scenery, I recommend you check out their freeware airports that are within that scenery and add them as well.
As for the payware airports by Orbx, they're all great but I find the one I fly out of the most is the free one, KHQM, Bowerman. it also depends on the airport size and what you like in terms of atmosphere. The one that takes up the entire north end of one of the San Juan islands is nice, but it's kind of hard on my frame rates. For a medium size airport I like KBVS (Skagit). I have all of their airports and you really can't go wrong with any of them. I would just tell you to consider which aircraft you enjoy flying the most and what kind of airfield they operate out of and look at which of those airfields fits your flying style the best. Oh, and definitely make sure you get the freeware 7WA3 Westwind as well, which actually includes three free airfields that will challenge your flying skills.
P.S. - If you ever get their NRM add on, make sure you get the freeware Canadian airports as well. They're really cool small airports. Well, actually, just buy them all. Eventually. Your biggest problem will then be figuring out which airfields you want to fly to and you won't fly anywhere else. Except other Orbx scenery.
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