the most powerful i have ever seen him was in the first secret wars after devouring the sun. but i on't believe that was his full power or potential. when he absorbed the dreaming celestial it actually satiated him for a short time but then the hunger started again almost immediately.
to answer your question -- i don't think he's ever been shown at full. maybe earth x some time?
There is no such thing as Galactus at 100%. He has unlimited absorption capacity - he could eat the universe and not be at max capacity. But Manjaro is right - we usually only see him when he's attacking a planet, and he's usually only attacking a planet when he's hungry. But I've seen him at his best right after Morg found him a great planet to eat to prepare for his battle with Tyrant.
Amazing! I was just going to post a subject line on this very thing. My feeling is that a "full powered Galactus" is a myth. As stated above by DEMIGAWD he has an unlimited capacity. This is shown in the Avengers saga concerning Nebula and the Kangs, where sometime in the future big G has become a universal black hole consuming EVERYTHING. I'm sure at this point he power level was far beyond anything (other than LT and Boss by necessity) and still growing as he continued to devour the universe.
So please pass it on to Everyone that all threads involving a "full powered Galactus" should cease and diciest as there is no such thing in Marveldom
Thank you all, have a good night, be safe and drive safely...it's crazy out there!