Thanos has a tendency to delegate the dirty work, so I don't really see him as being the most powerful. He's kinda like Trump...he has the potential, as a celestial being, but he's too lazy and full of himself to really achieve his potential.
Apocalypse is a mutant with exponential resilience and even greater ego...but he is far from having celestial level power. Meanwhile, Galactus devours entire planets for breakfast...quite literally. The Beyonder is an entire universe who decided to play a game of chess with all of the interesting toys in his toy box. Those interesting toys being many of the most powerful heroes and villains of the Marvel multiverse, including Galactus.