It shouldn't be hard to guess who it is, either. Minecraft creator Markus "Notch" Persson was invited to play in Team Fortress 2's third Mixup event. Playing on RED, he was teamed with Valve's Jon Lippincott and Cobalt's Daniel Brynolf (against a BLU comprising filmmaker Freddy Wong, Valve's Robin Walker, and others). After Notch collected the "Primeval Warrior" medal in a later game, Walker also presented him with the "Top Notch" hat.
As described by the Team Fortress 2 wiki, the hat is a cardboard box that resemble's Notch's avatar in the visual style of Minecraft It's equippable by all classes (as this gallery shows) although because of its size, "it significantly clips with items that exist in the whole head, necklace, glasses, ears, face and beard equip regions."
As of now, Notch is the only one to possess it. From "Top Notch's" description:
Welcome to a secret society so exclusive it makes the Illuminati look like a Costco. How exclusive? You are the only member. This is the only item of its kind in all of existence. So don't craft it, Miney.
In case you're thinking Notch doesn't have skills deserving of such a trophy, check out this box score from another match, in which he was the overall high scorer, also bagging 37 kills.
I Love Team Fortress 2 [The Word of Notch]
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"Top Notch"-equipped Demoman
"Top Notch"-equipped Engineer
"Top Notch"-equipped Heavy
"Top Notch"-equipped Medic
"Top Notch"-equipped Pyro
"Top Notch"-equipped Scout
"Top Notch"-equipped Sniper
"Top Notch"-equipped Soldier
"Top Notch"-equipped Spy