Grey Knights/YMMV

Base Breaker: The 5th Edition codex has a great many people wanting to murder Matt Ward by his retcons of the chapter, various internal What the Hell, Hero?, and his apparent losing of the plotline regarding Chaos being The Corruption.
Canon Defilement: Sweet Emperor of Mankind, even C.S. Goto had a better idea of how Warhammer worked than the guy who wrote the fifth edition codex.
Creator's Pet - Supreme Grand Master Kaldor Draigo. A few early achievements are as follows: Single handedly killed the daemon prince M'kar in his first combat action with east. Single handedly banished the daemon primarch Mortarion back into the warp after easily beating him to within an inch of his life and carving his predecessor's name into his heart. Took down a chosen Bloodthirster of Khorne in unarmed combat, stole its axe, and reforged it into a sword with only his mind. The Fandom treats him like a bad joke at best and a sign Matt Ward is a talentless hack trying to one up all other authors in Games Workshop.
Game Breaker: The Plasma Syphon. A weapon which makes any plasma weapon useless against a unit equipped with it. The problem was that some players began to take the fluff into account and apply it to anything based plasma. Complaints were made to Games Workshop who ignored these letters. When the FAQ was released amongst the many leaps on logic it was confirmed that the weapon made anything related to plasma useless. Tau players are now refusing to play anyone with Grey Knights as 95% of their weapons are based on plasma technology.
Magnificent Bastard: Alaric becomes this at the end of the Omnibus, seriously, who could have managed to fool not one, but three greater daemons along the series, or make a plot to take down an entire daemon world and the chaos lords at its command? And all of this while fighting through hell and surviving numerous Mind Rapes?