Best unique identifier to use on API requests for guests

I want to make a number of API requests from an iOS app to a Rails backend API. What’s the best unique identifier to send along with each API request to tie those actions to a particular guest, and why?

Some options:

I’ve also seen this message from Apple:

Starting May 1, the App Store will no longer accept new apps or app updates that access UDIDs. Please update your apps and servers to associate users with the Vendor or Advertising identifiers introduced in iOS 6. You can find more details in the UIDevice Class Reference.