Luke.
Sidious definitely grew in power between ROTS and ROTJ, but even according to Secrets of the Sith (written by Sidious during TROS), he always believed that Luke was a legitimate threat to his supremacy. It wasn't until after Luke's death that Sidious felt confident that he could carry out the rest of his plans.
Luke, hands down. Peak Luke knew how to commune with the spirits of the departed Jedi. He explicitly taught Leia that skill, who then passed it down to Rey. That's the same skill Rey used to destroy peak Palpatine in TRoS.
Even putting aside the potential for an "I-am-all-the-Jedi" style power amp, Palpatine always viewed Luke as the greatest threat to his power. Even as far back as Return of the Jedi, Palpatine was fearful of how strong Luke had become. By the time Luke hit his peak, RotS Palpatine would just be completely out of his depth.
Yeah, Luke.
At his 'peak-base', I'd put him somewhere between RotS Palpatine and RotJ/TRoS Palpatine... So basically > anyone outside of the most powerful canon character, ever. As mentioned, Palpatine always considered Luke to be a legitimate threat to himself(even after the RotJ-era) -- Palpatine wouldn't have held such a high opinion of Luke if he were some insignificant feeb that could just be finger-flicked from existence(like Vader or w/e.)
Though like UA said: it's also conceivable that Luke could have beaten post-dyad!!TRoS Palpatine(assuming *all* abilities were on the table), given that he would've logically had the "omni-Jedi haxx" at his disposal... But that's neither here nor there.