Is thanos the most powerfull in Marvel Comics?

Though Galactus is very powerful and is necessary to keep the balance of the Marvel universe. He is not the most powerful, though he may be the oldest as he comes from the universe that came before. He managed to survive the Big Bang, which is proof of how powerful he is.

However, there are many beings that can be and are far more powerful then Galactus. To a varying degree the Celestials, a race of cosmic beings from another universe can be considered more powerful. This is the race that essentially engineered the metahuman aspects that give many heroes their powers including all of Mutantkind.

There are then the avatars of various kinds such as Lady Death, Eternity and Infinity and Lord Chaos who are the conscious embodiments of various aspects (Such as the Universe, death and disorder depending on the character) of reality.

The Living Tribunal is perhaps the most powerful being that has been introduced in Marvel. When the character Adam Warlock obtained the power of the Infinity Gems The Living Tribunal answered to Eternity's anger because the Gem forged into a gauntlets called the Infinity Gauntlet granted Warlock the power of omniscience and omnipotence that rivaled Eternity's. In a fit of justifiable anger, Warlock attempted to destroy the Living Tribunal. But the Tribunal was unaffected.

However, the Living Tribunal did say that he served someone even more powerful then himself. A character that has never been introduced, only spoken of call The One Above All.

I hope this has not put Galactus on a smaller scale then you first saw him. Galactus's power is far beyond 99.9% of all the earthly characters.

A few misconceptions and facts about Galactus to clear up some common ideas:

Galactus, while portrayed as a villain at times is in no way evil. Though his actions may seem evil, he is simply a force of nature. The Devouring of life force from a planet is essence to the Universe. He is no more evil then a earthquake or a hurricane.

He has a daughter named Galacta who enjoys her time on Earth and even keeps a blog on the internet.

The Ultimate Nullify-er, a device developed by Mr. Fantastic (The Stretchy guy from the Fantastic Four) is capable of destroying Galactus in his weakened state of hunger.

Source: a life time of collecting and reading Marvel comics