Something, anything and everything !!!!
Generally always carry a basic toolkit, sockets, spanners, stanley knife etc, cable ties, lump hammer, spade, shackles, bridle, short lifting strop and decent towrope. Blanket, couple of wood blocks, zipup polybag for wellies, extra coat, hat gloves, spare workgloves, first aid kit, small fire extinguisher, cb and mobile phone. I also include the standard 27mm wheel nut, alongside a dual size socket for other 'normal' car wheels and extending bar (surprisingly good for a cheap thing!) I carry very few spares, serpentine belt (old one but good enough to replace a snapped one, even if it squeaks, rubber donut for rear prop, various nuts and bolts, a UJ, UJ nut socket, Hubnut spanner, and various other small bits, nuts, bolts, washers, bulbs, wiring and crimp connectors, insulation tape etc.
I also mostly carry a gallon of fresh drinking water, a can of oil, Duct tape, WD40, small bottle of brake/clutch fluid and anything else I chuck in the box, like a torch, small tie-downs, small tie-ropes/straps etc. Puncture repair kit, small compressor, ball of string
(You wouldn't believe how often i've needed this) England flag, extra gloves, wipes, cloths, a towel ...
When off-roading or laning with or without a group I add a Hi-Lift, more ropes/strops/shackles and a chain set made up with hooks to turn the Hi-Lift into a winch .. can't afford a decent Tirfor so this'll do ...
I also add extra wood chocks/a small plank, a couple of rebars and a Marquee tent peg (about 3' long with a good head on it) to use as a ground anchor if necessary, small bow-saw and hand-axe (though that's lent to a mate and not replaced yet!). I often also take a complete change of clothes and shoes/boots and some means of cooking, small gas stove, pan & kettle and tea/coffee makings, cans of soup, bacon, buttered bread/rolls etc .. and a small pop-up tent and sleeping bag! ...
It's amazing how often the lists change though ... lend someone a thing and forget to replace it, swap something for something else and find you need it. The lists above are (I think) pretty basic really ...
I also always carry a Swiss Army penknife with me .... absolutely invaluable ...