Cryptographically random UUIDs (v4). Batch up to 1000 at a time.
UUID version 4 uses 122 bits of random data (128 bits total minus 6 fixed for version and variant). The odds of two v4 UUIDs ever colliding are effectively zero — you'd need to generate about a billion per second for 85 years to have a 50% chance of one duplicate. Generated here using crypto.getRandomValues.