Two stage trigger (for PAC and fun)
Weapon Release (obviously)
TMS Up and Down and Right (for setting and resetting SPI)
DMS Up and Down (For zooming TGP and changing steerpoints - add left and right for switching weapons and sharing SPI if you have enough hats)
Nosewheel Steering (For NWS and laser and refueling)
Trim if you have enough hats (Trim can be handy, but I went a long time without using it, but it's really useful once you learn it)
AutoPilot 3 way switch and toggle button (I use this all the time)
China Hat Forward and Back (constantly used for everything)
Slew mouse (TGP/TAD/MAV everything)
Master Weapons Mode (single button to change between weapon modes)
Coolie Hat (for changing SOI - three way hatswitch basically)
For what I do those are the critical buttons I use all the time. Others are ridiclously handy to have on HOTAS, like gear, flaps, speedbreak, CMS, zoom, mic switch, lights switch, but maybe not critical.