The highest shiny rates in the series, in Ultra Sun and Ultra Moon.
In Ultra Sun and Ultra Moon, the Ultra Warp Ride takes you through wormholes to other worlds. Ordinary Pokémon you meet there can be forced shiny, and the chance depends on how far you travel and how many rings the wormhole has.
Wormholes with no rings stay at 1%. Ringed wormholes climb with distance, and the best case is 36%: a three-ring wormhole after travelling 5,000 light years or more.