It's a sad indictment on the current state of DC Comics that this list doesn't just consist of 'Superman'.
Why?
Oh, how awful! There are characters more powerful than Superman! And some of those characters who have also been around for decades!
I'll tell you why - Superman should be at the top of the tree, or at the very least very close to it - and he would be if not for the constant reboots and retcons and 'crises' he's been put through over the years.
They build him up and build him up and build him up and then have another crisis to put him back to struggling to lift a Volkswagen. It's disrespectful to the character to saddle him with writers so devoid of imagination that they can't think of challenges for him that don't involve rebooting the character every five minutes.
Supergirl gets it even worse, she's been repeatedly wiped from existence entirely.