In terms of tech/science
Galactus' ship Tea II (which he built) is bigger than a solar system. Dr. Doom stated he could conquer entire galaxies with just one piece of non descript tech in Taa II
He has a room dedicated to just sound. He has tech that recorded the "noise" produced by the big bang.
Galactus' tech (not Doom's) allowed Doom to usurp the powers of pre retcon beyonder
G's tech is beyond Reed and Thanos' understanding
Galactus had something to do with the ultimate nullifier's existence
He created Tyrant from scratch
His smaller ship was able to outrun the power of the IG for a bit
Etc
Even before he became Galactus, Galan was one of the foremost scientists in the "most advanced civilization in the universe"
Yeah I'm not sure why Mxy is even on here. He's been outsmarted by Superman as well. He is powerful, but he's not necessarily super intelligent.
I also don't see Darkseid as smarter then Galactus. Remember Darkseid has other scientists and stuff creating all the tech he uses. I'm also not exactly sure if Brainiac is even above Galactus.
Thanos is smarter than Galactus for sure imo. Other ones are harder to tell but I'd wager on Brainiac, Darkseid, and Metron being above him too assuming Galactus doesn't get his cosmic awareness.
Edit: Also depends on what you mean by intelligence. If it's just raw tech and science then I don't see any of these guys topping Galactus.