Mine would have to be League or Warframe tbh.
Binding Of Isaac Afterbirth - no other game I've ever played has made me so suspicious that the developer hates the player almost as much as they hate the main character. I've rage-deleted this game off my PC more than any other - which says a lot.
There's also Jagged Alliance 2 and all it's mods - inexcusably shallow world-building, a threadbare, low-stakes plot that nosedives in early game and never recovers, awful political subtexts and badly broken game mechanics (despite the miracles modders have managed to achieve with it) - yet it still (somehow) manages to be one of the best turn-based squad-based affairs around that inexorably tantalizes you with what it could have been.