Best Identifier for iOS device

I'm pretty new to iOS development, but I'm working on a small iPhone app for work. Every so often, the app will send an HTTP request to a server containing some information in JSON format. One of the things I would like that JSON packet to contain is an identifier so I know who sent the data to the server. I could create a login page for the app, but ideally I'd like to keep the data as anonymous as possible. I was thinking it might be better to store the device serial number (is that a thing) or MAC address or something unique about the phone as the JSON id, without resorting to someone's actual name. So, my question would be... what is the best unique identifier that I should use to keep the data psuedo anonymous. Also, I'm using Swift, so any places to look for code snippets to help me get started would be greatly appreciated. Thanks.