Of course, the GPS receiver. Can't live without it, right?
Reserve batteries. Always have some.
Pen, pencil, otherwise known as "logging instrument". Some caches may not have one inside.
Some items for trade
A compass. Though not required, it may come in handy. And it works without batteries!
Flashlight. Not just for searching in the dark, but perhaps you want
to try a night-cache.
Consider bringing a UV flashlight as well.
A smartphone is a versatile tool. Add internet access and it's almost unfair ...
Maps. Don't need one? With all those modern gadgets? Are you sure?
Don't forget what's most important: a brain