Well originally, Darkseid was supposed to be meant to be on Galactus' level, but as time went on, he didn't have enough feats to prove this, but it was still accepted. Crossovers don't count on this forum, but it doesn't stop the idea that both Marvel and DC pretty much agreed on what a confrontation between the two of them would look like. Galactus was easily the more dominant of the two. I have to also add that anything that sees print is canon, unless an official retcon is passed.
When Odin fought Galactus, Galactus wasn't at his higher levels, so you can't really get an accurate estimate there. Currently Donny Cates seems to be ignoring the fact that Galactus is no longer the Devourer, and is now the Life-Bringer. In current continuity Galactus has moved up 2-3 spots on the cosmic power scale enabling him to defeat Abstract level opponents that he was unable to defeat as the Devourer (Master Order, Lord Chaos), and do so with near ease.
Thanos is still a huge question mark in terms of his upper levels of power. We do know that at a minimum, he is able to destroy High Herald's with ease, but then again he was always able to do that. If Marvel suddenly placed him up against Zeus, Odin, The Stranger, or any others on the level, and he was able to defeat them it wouldn't surprise me because of how well he did against Thane.
For now I believe that they would defeat Galactus the Devourer, but lose a lengthy battle against Galactus the Life Bringer.
Can you explain this? I don't understand, because I thought that Galactus was dominant in that encounter. The Surfer on the other hand was about to get worked.
They were peers. Both had dominant showings against the other. A fraction of Darkseid raw power threatened Galactus life. Galactus nature established by Byrne making him resistant to being wiped from reality countered the Wipeout.
And that was a far weaker Darkseid than the present per Byrne as he absorbed multiple pantheons power since then.