The fastest prayer xp in OSRS is 1 ticking superior dragon bones at altars which can earn 1.3 to 1.7 million xp per hour.
The chaos altar is the fastest xp since you can unnote bones outside and also you spend more time at the altar since theres a 50% chance of saving a bone.
Although, theres a good chance of being pked which will affect your xp per hour, so for the best consistency, 1-ticking superior bones with a bone runner in a player owned house is the fastest per hour.