This is a tricky one, since Iron Man has technically gone through a number of different designs over the years, as the outfit is always evolving. It is difficult to pinpoint precisely which armor is the best, so we have decided to pull back a bit and note that the more general "yellow and gold" design is what we're going to celebrate here. You see, when Iron Man first debuted, he wore what looked to just be a suit of big, bulky, gray iron. When he became a regular superhero, Marvel tried to fix that design flaw by literally just spray-painting the armor gold. "Oh no, he's hideous!" "Oh, never mind. He's spray-painted gold. It looks great now!" Then Steve Ditko designed a new armor that worked red into the design and it really clicked.
Mike Zeck came up with the actual look and it is a stunning piece of design. All black except for the prominent white spider. Rick Leonardi also did some tweaks to the costume and it soon became the most controversial, yet surprisingly popular costume change in comic book history. Fans were aghast at first but then really started to enjoy the new look. Eventually, the design was adapted for the villain Venom, with Spider-Man returning to his classic look. It is popular enough, though, that it still makes occasional comebacks.