Diablo 4's hardest Capstones might be the best place to farm Legacy Uniques
The Pinnacle versions of Season 15's Prime Evil fights are proving surprisingly rewarding.
Diablo 4's new Legacy Uniques do not have their own dedicated boss loot tables, but there is a particularly good place to farm them once you reach Torment 10.
The Season 15 Capstone dungeons are a really good source for Legacy Uniques, especially for items like 
Leoric's Crown and 
Stone of Jordan. They can also drop Mythic Charms, so it is definitely worth adding to your farming rotation if you already have a build that can comfortably clear T12.
Pinnacle Capstones Can Drop Legacy Uniques
Season 15 brought nine Legacy Uniques back from previous Diablo games, including Leoric's Crown, Stone of Jordan, 
Henri's Perquisition, 
In-Geom, 
The Furnace, and more. But unlike regular Uniques linked to specific Lair Bosses, these items are not assigned to a particular boss, meaning they can generally drop from anywhere.
That makes finding a reliable way to target them particularly useful.
The three new Capstone dungeons give you exactly that once you reach the higher Torment tiers. Season 15 added Capstone-style encounters that recreate iconic fights against Baal, Mephisto, and Diablo, and they all have a Pinnacle version that becomes available at Torment 10 and above.
At that point, the dungeon icons turn red to indicate their Hell difficulty, and the fights become significantly harder than their normal Capstone versions.
The rewards are really worth the effort. We saw more than one person reporting regular Legacy Unique drops from these Pinnacle encounters, and Leoric's Crown and Stone of Jordan are the two that seem to show up frequently. There are also reports of Mythic versions of Legacy Uniques and Mythic Charms dropping from these Pinnacle fights, but the exact drop rates are not currently clear, so this really is just based on reports from different players.
You Can Also Farm Your Missing Soul Splinter
The three Pinnacle dungeons have another obvious reason to run them since each boss can provide its corresponding Splinter of Evil.
Baal drops the
Splinter of DestructionMephisto drops the
Splinter of HatredDiablo drops the
Splinter of Terror
This lets you switch to a different Prime Evil power after completing the seasonal questline and your reputation board, and it also gives you a reason to keep running the corresponding dungeon. So if you are already looking for a particular Splinter, you can potentially combine that grind with your hunt for Legacy Uniques.
Just Do Not Jump Into Them Too Early
They are called Pinnacle fights for a reason.
The Torment 10+ versions are much more challenging than the regular Capstone versions, so even if they are unlocked, it does not necessarily mean you are ready for them. A good sign is if you can already clear normal T12 content or not. If you can farm Helltide, Hordes, or Tributes and any Lair boss in Torment 12, then yes, you can probably take on these Pinnacle Dungeons.
Once you are strong enough, these three dungeons are a really interesting option for anyone who wants to target the Season 15 Legacy Uniques.