This is the tool box I have at a race meet. The part you need, is always the one you left at home.
- good quality hex set as has been mentioned - also an electric driver - handy when you are in a hurry between qualifiers and need to take the car apart.
- shock and diff oils
- spare tires and glue
- Bearings - lost of them - every type you use
- Thread locker
- General spares for consumables - fuel line, clutch parts, exhaust gaskets
- spare servo
Basically when something breaks, I by one replacement, and one spare for the tool kit.
I believe however that prevention is better than cure - after each meet the car comes apart and everything is checked - I use thread locker on screws and replace bearings BEFORE they are stuffed.