CBA
ACE
RHS (AFRF + USAF)
TFAR or ACRE
TRYK
^
These are what I personally consider 'essentials', though RHS is debatable.
Without them, I wouldn't even touch Arma.
This is my current list:
I am too lazy to write them all down, but if you google the abbreviations, you'll find out the exact mod.
All but '@gc' are available, that one is my units private mod with some fixes, extra gear and uniforms etc.
For UK Forces, as others mentioned, CUP should be what you're looking for.