Powerful is one thing... best is another.
There's a fascination with making the most "powerful" superhero... but an all-powerful hero makes for terrible storytelling. Which is why, for a new series, they had turn Goku into a kid again... why DC had to invent Kryptonite and why Marvel had to rip the adamantium out of Wolverine... why the Hulk has to be evil... and why Spiderman and Batman outmovie them all.
(Hell, the original Firestorm could kick all their butts with his transmutation powers... instant kryptonite! Transmute the air in everyone's lungs into cyanide! Muwahahaha!)
The best superheroes... meaning they do the most with what they've got... have got to be Captain America, Spiderman, Batman and Green Arrow. But Green Arrow is too fringe, Captain America has been hobbled by years of poor writing and lost relevance, and Batman is too... perfect.
So, it's: Spiderman.
Yes, he has super powers... and the spider-sense ability has become so overblown that they've had to come up with ways around it... but here we have a regular guy working several jobs just to make ends meet moonlighting as a superhero. And unlike Batman, who is on friendly terms with the police, or Superman or Captain America, who are icons, he's misunderstood, feared by some and hated by others.
Even when he loses his powers (can't recall which issue... during this arc he has a grand fight with The Lizard over the rooftops), without martial arts or military training, strength or high-tech equipment (beyond his web-shooters), he still tries to take on foes who could crush him flat in an instant.
In the end, I pick him because he's cool, he's believable and he's believably human. While many have tried the "flawed" superhero format, none get it quite as right as Spiderman.