What is everyone's must carry item be it in the field during training, deployment, or even activations?

So a fellow veteran amd myself were talking on the range the other day and we discussed our attire with professionalism and jokes. However this brought up a fun question.

What is everyone's must carry item be it in the field during training, deployment, or even activations (state active duty for those in the guard)?

Mine is and always has been a baseball cap. I have one hat that i have always worn since my first deployment and take it with me everywhere.

Yes i keep my K-pod in a pack and near but i love my baseball cap and wear it when i can. However, it goes with me everywhere. Along with a picture of my wife and kids, but I'm talking about objects like my cap for example.