Palpatine's Spirit feats don't prove he is the strongest Sith of them all
People constantly use this reason as to why Palpatine takes the leash when it comes about all the Sith Lords in history.
Let's settle things:
1. Palpatine didn't escape Chaos on his own - he was guided back to life by Sith spirits.
"NO! NOT THAT ... YOUR SPIRITS GUIDED ME BACK TO THIS LIFE WHEN I WAS DESTROYED BY VADER ... AND BY HIS CHILDREN ... MY AGING CLONE BODY WILL SOON DIE. I NEED HEALING ... NOW." - Palpatine to unidentified Sith Spirit on KorribanFrom (Dark Empire III: Empire's End)
2. Palpatine didn't journey thoughout space for 1 year. He was residing in Chaos for 1 year.
He had spent over a year disembodied, formless, drifting through the maddening void of the DARK SIDE.From (The Dark Empire Sourcebook)
And this is not a feat. Because that's what happens with all the spirits of evil beings.
3. Palpatine didn't cross the cosmic dark as spirit, he simply used a small Force storm to migrate his essence into Jeng Droga.
Even as he perished, Palpatine used the dark side knowledge the Sith Lords had granted him earlier to REND space itself and TRANSMIGRATE his essence across lightyears to Droga's body.From (Star Wars Gamer 5: The Emperor's Pawns)
4. Palpatine being dragged by the Jedi spirits in the after-life does not prove Palpatine's mighty strength in the Force.
You see: at that time only Palpatine was the most important big bad in Star Wars. So that's why the Sith/Dark Side spirits brought Palpatine back to the physical plane and not other Sith Lords. When Palpatine was finally killed, Empatojayos Brand took Palpatine's spirit in the Netherworld of The Force, not in the Chaos. Big difference. And there, Brand and his fellow Jedi made sure that Palpatine won't escape ever again.
It's known that Force users can channel Dark Side spirits by using objects or relics or something that belonged to the respective deceased evil Force sensitive.
Example such as when Marka Ragnos' spirit was summoned from Chaos by Tavion Axmis and the Disciples of Ragnos by using his own Scepter. That happened even before when Ragnos' spirit was unintentionally summoned from the dead by Exar Kun and Ulic Qel-Droma with the help of a pair of Sith talismans.
Chancellor Palpatine spent many years studying ancient Holocrons to learn the secrets of the Sith. The Holocrons enabled him to CHANNEL Sith spirits, who taught him how to harness dark-side energy and release lethal bolts of lightning.Source: The Ultimate Visual Guide: Updated and Expanded