Metal Cap 
(I have found there are really no good pictures, if any pictures at all, of the actual "Metal Cap", so I had to post the "Green ! Box" instead to represent this item)
First Appearance: Super Mario 64 (N64, 1996)
Latest Appearance: Super Smash Bros. Brawl (Wii, 2008)
The Metal Cap was one of the strongest items in Super Mario 64, and one of Mario's strongest upgrades below total invincibility from the Power Star in the Mario franchise.
In its first appearance the Metal Cap boasted many abilities on par with the Super Star, including invulnerability, instant death to any enemies touched my Mario, ability to sink underwater, and no longer needing to breath underwater. The only two things that could in fact kill Mario was fall damage, and falling off the stage itself. In Super Mario DS, instead of Mario, Wario was the user of the Metal Cap (albeit through another item called the Power Flower), and a few more abilities of the Metal Form were seen, being able to walk on Lava and Frozen Water without damage (although if Wario sinks below the top of either one he will instantly die regardless).
After Super Mario 64 we never actually see the literal "Metal Cap" again, however Metal Mario does go on to remain Mario's strongest form, taking the metal sheen many times when Mario is required to be a boss or secret unlock, as in Super Smash Bros., Mario Golf 64, and Dr. Mario 64, and was used in part of his strongest move in Mario Strikers Charged Football.
More recently, the Metal Cap (now known as the Metal Box) is a special item in Super Smash Bros. Melee and Super Smash Bros. Brawl, allowing any character to wear it, gaining many of the same abilities Mario himself held in Super Mario 64, although much heavier and not quite as invulnerable to damage.
One last thing I'll make note of is the special music that plays in Super Mario 64 whenever you used the Metal Cap. Not only was it the only item in the game to have its own song (the Wing Cap and Vanishing Cap both shared the same music), but the song itself was very well done and really accented being made of metal well (see below if you want to take a listen!):
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