Let's get this one out of the way first, because it's obvious.
Spidey's original blue-and-red crimefighting costume will forever be a classic, but the black suit was the perfect next step in the character's evolution. No longer a kid playing at superheroics, the black suit Spider-Man was battle-tested against serious foes, even taking to the stars to represent the good guys.
The black suit represented a Spider-Man who was evolving, taking on adult challenges, and yes, occasionally dabbling in darkness (and looked badass while he did so).
As the black suit saga continued and the suit's true intentions revealed, the true mettle of Spider-Man came to light. He resisted the temptation of power and violence, rejecting the suit's gifts and proving himself a hero.
As a consequence, he made a dangerous enemy by doing so, as the suit bonded to Eddie Brock and Venom was born. Spider-Man survived one crucible, and would soon endure another.
Author, puzzle guy, and lifelong consumer of pop culture. I'm a nerd for wrestling, Star Wars, literature, trivia, and all sorts of other things. Feel free to mock and/or praise me and my scribblings at @glennmandirect on Twitter.