A useful toolkit will contain versatile tools that can ideally do more than one job each.
Standard toolkits are a great place to start but can be tweaked to suit your needs. A proper kit doesn’t need to expensive. Cable ties and a few metres of strong duct tape wrapped around a lolly stick has just as much chance of getting you home as a fancy ratchet spanner that you can show off to your mates.
A torch, a multitool and an Allen key that fits most of the fittings on your bike is an excellent place to start.
A pair of pliers can be an invaluable resource, and if you’ve got space, it’s worth including some. Mole-grips are an alternative.
A puncture repair kit, either the full roadside bodgers set or one of those tyre sealant kits are a good choice. The latter has its disadvantages but comes with the gas you need to reinflate your tyre.
Screwdrivers are great. Try to have at least a Phillips and a flathead in your kit and make sure they fit the fasteners on your bike.
Adjustable spanner. Again, the size should be relevant to your requirements, check that it will fit most of the nuts on your bike.
A length of electrical wire can come in handy in an emergency, a couple of metres should do the trick. Don’t go for a thick gauge, keep it simple.
It can be worth investing in a decent tool roll too, there are plenty to choose from out there, but as always, make sure that the one you want is suitable for your needs.