The definition of 'worst' means irritates the most with regards to a Reddit-vote-based system using the word 'worst'.
So, obviously most people are going to be irritated by things that make most people feel that this individual feels undeservedly above most people.
It's a pretty logical profile-type to end up as the most irritating. I actually have found that usually the guy/girl who thinks he/she is the smartest in the room actually is usually among the top 20% of people in that room but not necessarily is the smartest at all.
The idea that smart people keep their mouth shut and stay humble comes from an overexaggerated interpretation of Dunning-Kruger effect. Not all geniuses fail to realise what they are and not all idiots don't know they are, either.
Put me in a situation where social skills matter, I will seem like a flailing fish who is struggling to stay mediocre. Put me in a situation that involves fast calculations and both deceiving and overcoming deception and you'll find a groundbreaking genius.
Einstein was spot on with the fish and tree-climbing analogy, it's just a less applicable one to this scenario as I'm talking about me in particular.