Which is more future proof for external HD, USB-C or Thunderbolt 3? I'm considering to get a new external HD for my Mac setup. While I have an old MBP 2013, I'm planning to buy a new Mac this year (possibly iMac).

Which is more future proof? USB-C or Thunderbolt 3?
USB-C it's the physical interface. A USB-C connector can hold either USB (up to 3.1) or Thunderbolt interface.

USB 3.1 has a data signaling rate of up to 10 gigabits per second, whilst Thunderbolt has a data signaling rate of up to 40 gigabits per second.

The faster Thunderbolt connection will ensure a more future proof hard drive.