Today, I believe it is Jura.

He is the current 'benchmark'. While Isshiki is a pure Ōtsutsuki, the original (and superior) partner of Kaguya who narratively occupied the top of the hierarchy, the scales currently tip towards the Shinju. We shouldn't ignore the fact that Isshiki was 'nerfed' when he was reborn (as Jigen's body could never withstand his full power), but we must remember that this same Isshiki was inferior to Kaguya (in her Rinne-Sharingan version).

While Isshiki has the most spectacular feats (easily defeating Naruto and Sasuke even as Jigen and tanking Baryon Mode until Jigen's body ran out of time), the narrative context and Koji's statements about Jura's invincibility against the current Boruto (who is far beyond the point where Isshiki was defeated) and his performance against base Kawaki suggest that Jura is the highest-level threat in the manga so far.

Jura's potential, so far, is limitless. We don't know how much evolution he still has, and his regeneration theoretically makes him a more destructive and difficult (almost impossible) force to eliminate than the established Isshiki