Best way to get exact proximity between devices
BLE, RFID etc, these are all ways to exchange data, they're unsuited for location purposes !

Sure, EM waves take time to cover a distance, but take a moment to calculate what time we're talking about, it's a couple of ns. So your devices then need to be time synchronised by a couple of ns.

But it works for GPS ! Yes but what is the distance and accuracy there ? The distance is hundreds of km (distance between you and the satellites) and accuracy is 10m at best.

Signal strength is also not very well suited to measure distance. Sure theoretically it is easy but in practice it is not. The actual radiated and received power is not easy to measure accurately (and almost impossible with cheap equipment). Signals also reflect against other objects.

What might work is a device that transmits and receives at the same time, short range radar based solutions do that. Cheap short range radar solutions do not exist yet, but we're working on them ;-)

At the moment you're stuck with ultrasound based solutions.