Who is the biggest badass of Marvel?

Definitely Galactus. And yes, he is that powerful. He's arguably the most powerful supervillain in comic book history.

Galactus has literally devoured entire planets, and has fought countless superheroes to a standstill. In the movies his portrayal was laugh able to say the least, but in the comics, he is truly unstoppable. There is no force that has ever killed Galactus, only forces that have made him retreat.

I'm not familiar with Transformers, personally didn't enjoy it so I can't compare for you, but from your description I'd have two answers.

1) Jean Grey/Phoenix/Dark Phoenix - In the Dark Phoenix saga Jean Grey was taken over by the Phoenix Force (highly recommend reading it) and that gave her the power to fly to another galaxy, consume one of its suns (which killed 5 billion people) and single handedly take down all of the X-Men. In later comics she has used the phoenix force to resurrect herself and others. The Phoenix force is supposed to predate the universe and pretty much be an entity that is life and death with all of time and the universe to explore. Jean Grey is the only mutant known to have been able to somewhat control it, everyone else who has been in contact with it has been overpowered and more or less become a slave to the Phoenix force. Jean Grey/Dark Phoenix is not even defeated, she sacrificed herself by using her telekinetic powers to make Cyclops' optic blasts kill her.

2) Thanos with the Infinity Gauntlet - When Thanos finally gets the Infinity Gauntlet he becomes essentially a god, with a snap of his fingers he erases half the sentient life in the universe. He is challenged by the surviving heroes who are defeated (and for the most part killed). After defeating various others who oppose him he leaves his body and becomes the living embodiment of the universe. While he is the living embodiment of the universe his physical body is left vulnerable, and his granddaughter Nebula removes the gauntlet from his hand and uses its power to undo all the horrible deeds he had done.

Both of these characters are very powerful because the powers they wield (Phoenix Force, Infinity Gauntlet) can only be controlled by incredibly powerful people. In the case of the Phoenix Force only Jean Grey is known to be able to control it, and the Infinity Gauntlet is made of the Infinity gems which individually require an incredibly powerful character to wield them, so to use all of them at once takes someone of enormous power! These are the only characters known to have used the Infinity Gauntlet and not died as a result:

Thanos

Nebula

Adam Warlock

Magus

Spider-Man

Mr. Fantastic

Professor X

Darkseid

Iron Man

Captain America

Doctor Doom

Black Panther

Hopefully this answers your question...