Where are you hunting? And for which Species? My personal preferences:
1. California or Valley Quail- #7 1/2
2. Gambel's or Desert Quail- #6. Hunting these guys, you may encounter some open country and they will flush a little further out than Valley Quail.
3. Mountain Quail- #4- They are a little bigger than the others and having to take 50 or 60 yard shots isn't that unusual. Plus, the heavier shot will break bones at that range.
4. Mearn's Quail- #7 1/2. Don't forget the dogs! These guys will let you step on them before they flush.
5. Blue or Scaled Quail- # 6 for me. They are tough little buggers and run like antelope...
6. Bobwhite Quail. Can they really be called quail? They don't have any headgear...