..Which version of Superman was the most powerful? Logic says All Star, but Red Son made short work of the Green Lantern corps....

I really don't know, there aren't many comparable feats. And considering this is an alt universe and the GL corps were vastly different (and featless), plus instead of the most willful in the galaxy, they are the most willful in the USA(huge difference in caliber right there), I don't think it is such a big feat.
Logic says All Star because he is supposed to be Superman on stereoids. Plus he had some nifty new powers and his super-intelligence was off the scale. But Red Son survived a super-nova in that last issue

Hard to say… All-Star Superman got quite a boost of power, but outside of his Morrisoned’ feats he doesn’t seem to me like that much stronger than mainstream Superman at his best. Red Son Supes also was portrayed as massively powerful through the entire Elseworld… It’s a draw for me imo.

The last issue had Morrison involved in some capacity,safe to say All-Star > Red Son.

All-Star Superman. Definitely!

The Red Son story never much appealed to me. And the end was so strange that when I finished reading it all, I was actually disapointed.

All-Star Superman at least brought something new and fresh. They mashed together influences of various versions of superman and made stories that caught my attention has a fan. Something I can't say about Red Son.