Most powerful superheroes? That has to be one hard question to answer. We each have our own interpretations of what being "powerful" is. That means one view on the most powerful superhero will differ from the next person's view on who the most powerful superhero is.
To me, I happen to feel that Batman is the most powerful superhero. Though Batman is an ordinary human being without any superpowers, it didn't deter him from taking on the most powerful heroes and villains. He is the living embodiment of taking the human body beyond its natural limits through hard work, persistance, and determination. Batman is proof that hard work and perseverance does infact pay off.
You could have a bunch of awesome superpowers, but you have to know how to use it. If you don't, you're not that powerful. Watching episodes of Justice League and Justice League Unlimited, Batman managed to take on major villains like Darkseid, Brainiac, etc. When he was captured in one episode of Justice League, he managed to outsmart just about every villain that was assembled under Lex Luthor.
It shows that a mere mortal with an adept mind like Batman can take on the biggest and baddest villains. In one episode of JLU, it was revealed that Terry McGuiness who plays the new Batman in Batman Beyond is actually Bruce Wayne's son through DNA manipulated by Cadmus which is one of the Justice League's biggest antagonists throughout the JLU series. Terry broke into Amanda Waller's house and they talked.
Those who aren't with JLU that played on Cartoon Network's Toonami, Amanda Waller is a United States NSA operative who is the head of Cadmus. They're creating and recruting superhumans to combat the Justice League. But in the future, they made up and Waller became an ally. Waller gave Terry the talk about what kind of person Batman was. She even knew his real idenity of Bruce Wayne the head of Wayne Tech Enterprises. Waller was fond of Batman's adaptability to just about any scenario even if the odds were against him.
She said that a world without a Batman worried her. That's when Cadmus took all sorts of measures to change the DNA of Terry's father into Bruce Wayne's DNA. So far it seemed very plausible because Terry McGuiness does in fact look like Bruce Wayne's son.
Being able to show that mere mortals can take on superhumans is one quality that makes Batman "powerful." That's just one of the meanings of the word "powerful."
Another example of a powerful superhero in my opinion is Wolverine. I am a very huge fan of Wolverine. Asides from the regenerative healing factor, he has an adamantium endoskeleton. That was a pretty good concept on how Wolverine was captured and experimented on with his endoskeleton being laced with adamantium which is the highly indestructable metal in the Marvel Comics Universe.
But Wolverine didn't depend on his healing and adamantium claws alone. He had a mind adept just like Wolverine. Despite the gruff looking exterior, Wolverine is highly intelligent as well. Wolverine's about over 125 years old at the moment. He's travelled the world, made many friends, and made many enemies. The man is very well cultured and well travelled. Seeing as much as Wolverine did, it was a good concept to mentally keep him grounded to reality.
Plus Wolverine has went up against numerous superheroes and supervillains as well.
In a nutshell, Wolverine is one bad mutha...
So far, these are two superheroes I think that are pretty powerful. Remember, "powerful" is a state of mind. One's definition of a powerful superhero differs from one person to another.