I honestly feel that Spider-Man has the greatest costume in comics. Here are my reasons in no particular order.
1. Perfect concealment. As a superhero who's trying to protect his identity, you can't go wrong here. No fingerprints are left anywhere. His hair, nose, mouth and even his eyes are completely covered. There could be a black guy or an Asian guy under there for all you know.
2. Supports his theme perfectly. Some characters' outfits tell you nothing about them. The spider motif is clear throughout here. A spider symbol on the front and back, webs on the face, webs on the chest-this is just the right level of theatricality and keeping with the name.
3. Has an air of mystery. In this case I'm referring to the hidden goodies like his web shooters and utility belt. How many times has somebody subdued Batman and the first thing they do is go for his belt? It's prominent, it's brightly colored, it's begging for attention. By contrast a casual observer wouldn't even know that Spidey is wearing one. I tend to think Ditko was trying to create the illusion that the webbing might be organic, keeping its source out of view.
4. No trunks. I really hate trunks.
5. Great use of color. Many costume designs are just solid from top to bottom, apart from the symbol and maybe the gloves and boots. I think this is what makes some people think the trunks are necessary to break up the color scheme. Here it's nicely distributed, with the web markings confined to the red areas. The red and the blue stand out in their own way, and no additional colors are employed that might make the uniform gaudy.
6. Distinctive through and through. If all you could see was Spider-Man's arm, you'd know it was his arm. If you look at his foot you know it's his foot.
All in all this is an intelligent design, which most characters don't even come close to.
