Diablo 4 Season 15 has a free Mythic quest you can repeat every week
Ogden's Sign gives you a guaranteed Harlequin Crest, and the quest resets regularly.
This one is part of the Old Tristram quest! If you did not do Ogden's Sign yet in Diablo 4 Season 15, there is a very good reason to make the trip to Old Tristram. The short side quest rewards a guaranteed 
Harlequin Crest, better known as Shako, and it does not require you to beat any hard boss or farm anything at all. The reward can also roll with Greater Affixes, making this more than just a free Mythic to salvage.
How to Reach Old Tristram
We have a small guide for you on this, but you do not need to complete the entire quest chain to reach Ogden. All you need to do is collect all 10 Journals, and you will be able to teleport to Old Tristram. Once you are there, head to the town's fountain and speak with Ogden to start the quest.
All you need to do is talk to all the NPCs in Tristram (this unlocks an achievement) before heading into The Labyrinth. This is Diablo's Capstone Dungeon, but you do not have to complete it. Everything is done on the first floor of the dungeon.
Inside the Labyrinth, stay on the first floor and look for a square-shaped room with a demon inside called Snotspill. Talk to him, look for another square-shaped room, kill the enemies, pick up Ogden's Sign, and teleport back to Old Tristram. You can go back to Snot, but he will just attack you, and that is it. Not worth the detour.
The whole thing takes around 10 minutes once you know where to go, and you do not need to fight the three Prime Evils in Uber Tristram to get the reward. When you hand in the quest, you get your Harlequin Crest.
The Quest Resets Regularly
There is an even better reason to keep Ogden's Sign in mind, because the quest appears to come back. The exact schedule has not been officially confirmed, but it seems to reset once a week on Tuesdays. This is also true for the Butcher quest in old Tristram that rewards 
Prime Rib.
Yes, you probably do not need any Shako, but it is still a free Mythic you can turn into a 
Resplendent Spark.