Anything within the realm of Steampunk/Victoriana up to Supers-level Sci-Fi, Modern/Supernatural crossover being my preferred mix. I'm the only one in my group that like Reeses' Cups, though...
My group only likes Dragon Age-style fantasy or Firefly-level Sci-Fi. There were murmurs of using a fan-made Mass Effect system created using FATE, but those murmurs died quickly.
And perish forbid they try anything too silly or juvenile. Gotta have their weekly dose of Grit.