Luke goes grey jedi in Return. The difference between light and dark powers isn't as clear cut as people would lead you to believe. Luke, Obi-wan, and of course Mace all tap into their anger during fights, which in theory is a 'dark side' power...

lol i was about to say that

Yes! Think about how visibly angry Obi-wan gets after Qui-Gon is killed. He breaks the Jedi rules and uses anger to chop Maul in half...
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Yea, and Ashoka force chokes a Trashdonian during the clone wars (S3E22). Grey jedi are my favorite, they accept the force as...a force, not just Light or Dark.

That's right. Light & dark is just how we view it. In the star wars universe the force is in everything. The lines between light and dark are not clear cut at all.

Although, in a way, the Sith are "more powerful" than the Jedi, because they can use all aspects of the force, not just the light. But I think that Grey Jedi are the "most powerful" because they use all aspects of the force, like the Sith do, but instead of their emotions affecting their actions, they let the Force guide them.

Yes. The grey jedi have the potential to be the most powerful. The jedi order routinely hurt their cause my viewing everything too rigidly. The sith are too blinded by hate and power. Only the grey jedi can follow the (so called) 'will of the force' and take advantage of most of its aspects.