Critical in that order:
Look up (button that you can press at the same time as using your hatswitch, to combine views!)
Fire Primary
Fire Secondary
Flaps Down
Flaps Up
WEP
Gun Zoom Toggle
Good to have:
Talk buttons (Range and/or Tunable)
Combat Trim (if you do not fly with autocombat trim)
Check6
(Select engine - if you fly multiengined planes a lot and control engines separatly)
Nice to have:
Clipboard Toggle
Chat Window Size
Gear Toggle
Engine toggle
Autopilots
.......
As far as the "Good to have" and "Nice to have" is concerned, thats somewhat personal preference. Watch yourself while playing, map the functions that you use in battle the most.