The short answer is, any tag that will work with your chosen device.
However, tags with 13.56Mhz and ISO 14443 or 15963 should work best with mobile phones and tablets. We've found that tags containing an NTAG213 chip inside have broad compatibility with Android and iOS devices.
If you are at all unsure about which tags to get, tell your tag provider which device(s) you plan to use, they should be able to determine which tags are compatible (some will even provide free samples to test).
There are also many form factors (shapes) and options. The most common ones we recommend are simple "button tags" for tours and "card tags" for user badges.
- Button Tags: A small disk. Most often used as tour checkpoints.
- Card Tags: The shape of a business card. Most often used as badges/User IDs.