thats a hard question since what one person considers essential another may not.

Short answer a flight box should contian the tools and items neccessary to support "flight operations"

Here's my list

1 Place for Fuel and pump
2 Power panel and 12 volt battery
3 extra props, rubber bands, glowplugs
4 wrenchs, pliers, allen keys, screwdrivers
5 xacto knife
6 volt meter
7 prop wrench
8 bandaides
9 AMA card (need it to get pin)
10 hot shot (glowclip as backup)
11 electric starter (chicken stick as backup)