There is no single answer but here are a few pointers:
1 do you want a medical kit or a survival kit???? totally different
2 if you want a medical kit a) who might be able to use it - a doctor, nurse, EMT, first aider, pilot
b) how many casualties might you have
c) how long do you need to treat between accident and rescue
d) what legislation is there to prevent certain items
Now you can start putting it together, but the basic areas are:
ABC resuscitation
Trauma management
Pain management
Burn management
Cardiac management (if in flight)
Medical management (if in flight)
and for each you have a list as follows
Equipment
disposables
drugs
storage
Many so called experts will give you advice, but caveat emptor