Palpatine also did the one thing Plaeigus could not and that was bring himself back to life. They had to kill him four times before he was done for good.
The only other Sith that may be stronger is Darth Revan. This next part is from Star Wars Wiki..
"In later years, Revan was revered and remembered as a very Powerful Force User in Galactic History. Many learnt off of his teachings and his actions; Darth Bane basing his philosophies and rules over many things done by Revan. Revan had many Cults devoted to him and thousands of years later, a Sith Lord came up with his new name by basing it off Revan; naming himself 'Darth Rivan'. "
Now that is power if you ask me. He was feared in his time and revered after it.
Palpatine is the culmination of the Rule of Two..
Each new sith becomes more powerful than the last, He was the final sith. Revan started the rule of two, every new sith after him was more powerful than the last :/ .
Vader should've been 200% stronger than palps but lost his limbs, therefore he was still the most powerful.