I ussually carry:
Assortment of screwdrivers
Assortment of wrenches, pliers, cutters
One medium sized ratchet set (with metric, SAE and Torx fittings)
Some large sockets (in case you need to disassamble an axle or such; my socket set does not carry these, so I need them as an additional)
Allen wrenches (Metric, Sae and Torx)
Tire repair kit
Water, Coolant, oil, ATF
Some spare hoses (vacuum, fuel, etc.; jsut enough to get you out of a bind)
Duck tape (I cant fix as many things as McGyver could, but it still good to get you out of a bind!)
Some chain (you'd be surprise at the uses you can give it!)
Recovery Gear (winch gear, tow strap, etc.)
Emergency gear (some dried food, a jacket, a blanket, etc.)
Large metal tube (it's a breaker bar, a lever, etc)
BFH
Spare parts (I ussually carry U-Joints, couplers for lockers the air lines, and some other small parts).
If you have OBA, you may want to include some key air tools, but I dont carry any. Same if you have an on board welder.
Basically, the rougher the terrain, the more chance of breaking, and the more tools you will need to fix those breaks... This is just kind of what I take myself, but your needs may be different.