Sith-wise in my opinion it really comes down to "Do you think thing is more or less OP than that ridiculously OP thing." Palpatine had Force Storms, Naga Sadow blew up stars, Exar Kun sacrified and entire people and came back, Nihilus ate planets through the Force, Sion literally could not die unless he wanted to, Caedus while not as OP as some of the above, was still ridiculously good at a ton of different things. Then you've got newer things like Abeloth and the Sith Emperor of TOR doing crazy things as well. Sith have some seriously BA dudes.
Jedi-wise has a lot less of the "Gods amongst Gods" type of guys, but Luke Skywalker, Thon, and Nomi Sunrider were all up pretty dang up there, with Luke Skywalker seeming to get a new trick every few months to add to arsenal. As much as like Revan, he's not quite up in this category, but he's pretty close. I wouldn't be surprised if there are some others I'm forgetting, but if you get into EU things can get pretty ridiculous pretty quickly.