That's a difficult question. Because fans of different characters would contend for their favorites to be the most powerful.
To that, when you’re reading different Marvel comics, you would find different superheroes/villains portrayed, on different timelines/multi-verses, as the most powerful.
For instance, when you look at Deadpool, you would think he's just like a regular guy with a bit of superpower who's just as good as to stop the bad guys (who even Ethan Hunt and James Bond can stop). However, in 'Deadpool Killology' he was shown killing literally every Marvel Superheroes out there.
So, per that comics, you can call Deadpool as the most powerful being in the Marvel Universe.
But there are, of course, many others.
By many, Doctor Strange is assumed to be the most powerful. The Sorcerers Supreme single-handedly defeated Dormammu, who is recognized to be equally powerful as Thanos, if not more.
Captain Marvel is another name that comes in this list of most powerful beings in the Marvel Universe. She slayed in Avengers: Endgame. And she was just as powerful in the comics.
(Courtesy: Film School Rejects)
Then there's Thor who, well, we all know about and his powers. We also have Scarlet Witch (Wanda), Beyonder,Phoenix Force, Magus, Odin and so many more names thatare still to be mentioned in the movies.
Of course, we cannot miss Thanos.
Heck, even Ebony Maw is known to be extremely powerful. So, it was really quite surprising (and amazing) that Spiderman managed to kill him (with his pop culture knowledge) in Infinity Wars.
That said, here are the popular names that people expect but won't find in this list of most powerful beings in the Marvel Universe: Iron Man, Captain America, Black Panther, Hulk, and Spiderman. Yes, they are all popular and super-favorite of all. But they are far from being the "most popular".
Here's who I would include in my list of 5 most powerful being in the Marvel Universe (very opinionated and in no order):