For me, it's a tie between One Directioners and those several thousand obnoxious PewDiePie fans who think he copywrites anything he touches. Both would kill you if you speak out against them. Seriously.
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PC Elitists, Bronies and Pewdiepie's rabid fan legion. This doesn't mean all people who play on PC, like MLP and watch PDP are idiots but when those three things are taken to the extreme you get the most irritating people on the planet.