I started selling all the invitations, feels like all "pinnacle bosses" drop trash this time around. But maybe it's the same in PoE1? I rarely do bossing in that game so I have no idea.
Xesht = Controlled Metamorphosis (Large radius with minimal negative rolls is extremely valuable because it lets you allocate both Kite Runner and Spagettification, also Chakra of Thought if your build can use it), Uul-Netol's Embrace
Ulroth = Life flask is big money and so is the Exodus gem. Omen gem is several divines still too.
King in the Mists = From Nothing is a few div if you hit the right node (Vaal Pact mainly, since it gives access to a bunch of movement speed that is otherwise way too far to path to). Ingenuity is mostly worthless now, but if you get a lucky vaal it can still be worth almost as much as a HH.
Simulacrum - Megalomaniac with the right notables is huge money. Most are worthless though, and it's stupid rare considering how many times the dudes that drop it spawn during a run.
Arbiter of Ash - Prism of Belief is worth 20+ div for unid. Or you can gamble on the very small chance of hitting +2/+3 to one of the handful of skills that are worth 50div to around a mirror.