The evolution of Iron Man from his bulky original suit to the current look happened over the course of several redesigns, including a gold version of the original, the "Silver Centurion" armor of the '80s, and Football Pants.
Oh, Sue Storm. I don't know what possessed someone to redesign your costume as a swimsuit with a giant "4" cut out over your cleavage. Oh wait, yes I do. As you might expect, the Invisible Woman's costume reverted back to the bodysuit that Jessica Alba later donned in the movies, and the "4" costume now lives on only in infamy.
By far one of the most absurd costume changes in comics history, Speedball was a happy-go-lucky teen hero who made colorful energy bubbles, until one of his teammates accidentally blew up 612 civilians. He renamed himself Penance, as now dresses in a metal gimp suit with 612 internal spikes that stab him every time he moves. You know, as penance.
In the '90s, Aquaman ditched his original costume in favor of an updated look with long hair, a beard and a hook in place of his left hand, because his Super Friends gear apparently just wasn't piratey enough.
After Bruce Wayne had his back broken by the villain Bane during the 90s, another vigilante took over the mantle of Batman -- while wearing brightly-colored battle armor and excessive yellow pouches. He also eventually started killing people, forcing Wayne to take back the Batman name -- and the costume we know and love.
After his secret identity was revealed in the 90s, Daredevil faked his own death, had a breakdown, became a street hustler, and returned in a mecha-suit with razor shoulderpads. Mercifully, none of this lasted, and he returned to his old identity and costume for reasons that ultimately, don't really matter.
One of the most famous costume changes in comics history, Spider-Man's black suit was actually an alien symbiote that bonded to his body during an intergalactic conflict called the Secret Wars, but we're pretty sure it was really put there because everyone was tired of drawing all those webs on his classic costume. He eventually removed the symbiote and returned to his classic costume, except for a brief period when Aunt May was hospitalized and he decided to dress in a non-symbiotic black costume to express the darkness of his soul.
While a lot of folks (including Jughead) went punk in the '80s, Storm's transformation into mohawked, leather-wearing badass was less about the music and lifestyle and more about going down into the sewer and having knife-fights with super-powered gang-leaders. And THAT is punk as hell.
The biggest major change from Wonder Woman's traditional costume into something with pants coincided with the rise of the Women's Lib movement, but unfortunately involved her losing all of her powers in the process, which is kind of a mixed message. Much like bell-bottoms, her groovy costume soon disappeared in favor of a more classic ensemble.
The X-Men have worn a lot of colorful costumes over the years, but it wasn't until Grant Morrison and Frank Quitely gave the X-Men a more paramilitary-inspired uniform that they actually made sense for the mutant fighters. The more realistic take was also adopted by the movies, where they donned appropriate fitted black uniforms rather than neon spandex.