For me, hal jordan.

If you’re talking pure power, various GL comics keep referring to Hal Jordan as “the greatest GL ever” and he even literally become one with the green light and was able to create his own ring out of sheer willpower; a task thought impossible.
However other notables include Kyle Rayner, who was able to tap into all the emotional spectrums, and became the White Lantern, who had control over life itself – something Hal could never come close to (though Hal did become a Black Lantern for a while, and held command over death) – and use all the different emotional powers/rings.
A worthy note is the new GL Jessica Cruz who is (imo) showing that she may have the potential to one day surpass him as she was able to defeat the Power Ring (which controls it’s wearer by fear) and became a GL of her own right, despite suffering from crippling anxiety that she battles every day, but from sheer willpower overcomes it and fights alongside the Justice League to protect the Earth.

Thx for mentioning Jessica Cruz! I forgot about her. Very powerful and she was one of my fav characters in JL vs the fatal five!

When he tapped into the emotional spectrum, he gained hold of all 10 rings and he couldn’t control it.

Hal Jordan