Well EU Luke and Palpatine were said to be the strongest but I think some could take them on
Examples
Darth Sion: He only died cause he chose to (he could've just kept regenerating)
Darth Nihlus: This guy could walk on a planet and kill it (he was basically hax)
The "Force Wielders" from Clone Wars (these guys were also hax)
probably missing a few might think of more later
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XBL GT: Demon Ninja X2
"There comes a time in every man's or woman's life where they have to make **** up to cover their ass." - Hazama