Senior superlative given to the biggest freak/weirdo of the class.
Recipient is usually not well known by many people other than a small group of friends, if any.
Most likely found spending their time reading, studying, or drawing tattoos on their arms with a pen.
Shane: yo, who you voting 'Most Unique' today?!
Joe: i don't know, this name doesn't look familiar, i'm gonna pick this girl.
Shane: yeah, me too. i've never even heard of her!