I’m traveling to bastide Armagnac in the fall. Is it best to fly to Toulouse or Biarritz and then drive?
"Is it best to fly to Toulouse or Biarritz and then drive?"
Best compared to what alternatives?
If you mean Labastide-d'Armagnac, it seems Bordeaux is the closest airport, although Toulouse or Biarritz aren't much farther. Where are coming from before and going to after - that will determine the "best" travel method.
Definitely check airfares to the various airports that are reasonably close. Sometimes the fares vary considerably.