Diablo 4's best Legendary rings start with cheap Jeweler caches
A rare Transfiguration outcome lets you keep an extra stat and turns cheap rings into endgame gear.
The top of the leaderboards in Diablo 4 Season 15 seems to have crazy gear, and sometimes they even have extra affixes on things like Amulets and Rings. For the Amulet, it is pretty easy as long as you have the proper Horadric Cube materials.
All you need is a stack of 
Kullean Tuning Prism and you can reroll a second Legendary aspect on your Amulet. This keeps it modifiable, and you can still transfigure it normally afterwards.
For the rest of your gear, transfiguration usually means it is the end of the road for you. But if you are trying to optimize your gear as much as the top of the leaderboards do, you will have to seriously take care of your Rings.
Push Your Build With Maxed Out Rings
Why Rings specifically, you might ask? Well, the answer is simply that Rings can come with crazy bonuses that also boost the Gems you slot in there.
And all you need is technically just the biggest stock of Ancestral Legendary Rings you can get, and 
Volatile Primordial Dust. Take everything to the Horadric Cube and transfigure them all. Most of them will end up being Unmodifiable afterwards, but in a stack of around 20 Rings, at least one should still be modifiable after Transfiguration.
Once you have a few Rings that you can still work on, check the bonuses you obtained. You want something that can be used. Gem Power is obviously the one thing you want the most, since it will boost the Gems you will slot into that Ring as well. But anything that can be useful for your own build, such as Cooldown, Elemental damage, Vulnerable damage, etc., is worth it as well.
Once you have a Ring with a really good additional bonus that can still be modified, you have to turn the rest into something that looks like it just dropped from a God. You can use different things to achieve that:
Chaotic Reroll
Focused Reroll
Affix Value Reroll
Enchanting at the Occultist
Add a Socket if it does not have one yet
Imprint the proper Aspect
Temper and Masterwork it properly
Once everything else is perfect and you have a god-like Ring, Transfigure it again. If you stay lucky, it adds another bonus and remains modifiable, and you can keep going. If not, you still got a second additional bonus.
Do not hesitate to use different Prisms as well, in order to narrow down the type of affixes you reroll for, such as 
Aggressive Tuning Prism, 
Protector's Tuning Prism, Protector's Tuning Prism, and all of these.
How to Farm for Rings
Before you can spend the rest of your day Transfiguring Rings, you need a stash full of those. Getting Rings is really not the hard part. You can use the Purveyor to Gamble on Rings, turn lower-rarity Rings into Legendary ones first, simply keep all the Rings that drop when you are farming Helltide or other content, or even target Rings during Kurast runs.
But one of the easiest ways is the Jeweler. You can craft Jeweler caches using Gem Fragments, and one cache drops 5 Rings or Amulets. So if you open around 4/5 caches, you get almost 20 Rings. Of course, only do this if you do not need your Gem Fragments.
If your build requires 2 Unique Rings, you will have to target farm the Unique Rings first, so that obviously would take much longer to achieve. We do not recommend you do this on uniques unless you already have a perfect unique with the affixes you need and everything. Then you can try to transfigure it and hope for it to stay modifiable just to get a few bonuses out of it afterwards.