Celeste and Perry
The Motive: Another clear one: we see Perry (Alexander Skarsgard) getting abusive with wife Celeste (Nicole Kidman), escalating to him actually slapping her and choking her. So the motive is there, and there are also a variety of hints that she's involved: toward the beginning of the first episode, when we're seeing flashes of Trivia Night just after the crime is committed, we see Celeste in costume, looking distressed and running hurriedly away. Then, in the last few moments of the episode, you can hear bits of recordings from the witnesses and a voice that sounds like Nicole Kidman says, "I'll just put my purse down here," both of which indicate that Celeste at least survives Trivia Night. Given that none of the other main characters are ever shown being interviewed in the police station, this seems like a big hint planted.