Practical costumes are hugely dependent on the power set of the hero and the situations they find themselves in. I've always thought the milennial Nightwing costume was hugely practical. It had armor, flexibility, stealth, gear compartments - all stuff Nightwing needs. On the other hand, Superman and Green Lantern would just have all that stuff get in the way; their skintight suits give them maximum flexibility while their power set provides the rest of their needs.
Coolest costume is tough. There are many whose designs appeal to me.
Stupidest costume is hands-down Hawkeye's purple scale mail. ::shudder::
The most impractical costume. Well, most supers sport a skintight leotard without armor, brightly colored, with no place to store gear. For most heroes, this is hugely impractical. Others sport giant capes and flanges, big shoulder pads, ornate headgear, and other outlandish features. And let's not forget that female heroes are traditionally clad in thongs and spike heels no matter the weather or terrain.
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