For me this varies a bit with the trip plans, weather, and the people I'm going with.

Generally speaking I always bring 3 sources to start a fire, 3 sources of light, a headlamp, a small powerful LED flashlight, and a very small LED keychain light, a complete change of clothes packed to make double sure they stay dry, and a couple days worth of extra food. I carry a katadyne pump filter and my backup is to boil water in my pot.

But I consider the above to be essential in anyone's pack and, except for the extra food, I use all these things.

For me personally, I carry spare reading glasses, sunglasses, and batteries for my GPS and lighting, and some light cord or rope. I almost never use these, so they are truly "Backups".

If I am hiking with others that have little experience I will carry some extra first aid items, a light weight rain jacket, blaze orange marker tape, Emergen-C vitamins, bug repellant, and more if I feel the need. I almost always end up handing a lot of this stuff out too.