I've read all the ones I can find (it's as simple as just searching words like '
stronger' or 'powerful' into
here).
He's adamant
in numerous interviews that Superman is top dog (even
saying that he's basically the most invulnerable), Shazam and Mongul are in the same league but not exactly as strong (with Shazam being equal only in Season 1), TK Martian Manhunter, Wonder Woman, Captain Adam, Icon, Aquaman, etc come a close second, and Hawk Man, Red Tornado and normal Martian Manhunter are near the same league.
The only claim I've seen contradict this is that
Superboy is > Aquaman. But that's contradicted by the actual show, IIRC, and Greg himself saying that Superboy is way less powerful than Superman.
He doesn't really seem to be much of a power scaling guy altogether, though.