Here's who I think has the biggest and least chances of dying.
Clarke: 0%: Eliza Taylor has been confirmed for the cast of Season 7.
Bellamy: 0%: Bob Morley has been confirmed for the cast of Season 7.
Raven: 0%: Lindsey Morgan has been confirmed for the cast of Season 7.
Murphy: 20%: Richard Harmon appeared at Comic-Con for Season 7.
Jordan: 10%: Since we just lost Monty and Harper, I doubt we would lose him so soon.
Echo: 40%: I hate to say it, but she might be the one. However, she is super OP, and this season went more into her past, which could mean, hopefully, they will keep her around.
Octavia Blake: 60%: If we lose a main character, it will probably be her. She is one of the most disliked, and I have a feeling that she might sacrifice herself for Bellamy or Indra.
Madi Griffin: 10%: She is still a child, so that might be a little much for a TV-14 show. Plus, she needs to get rid of Sheidheda before she can die.
Emori: 50%: Her fate is kind of up in the air, and she nearly died in the Season 5 finale, barely making it to the ship as Shallow Valley exploded, but she is a smaller character, and they may keep her around for the sake of Murphy.
Niylah: 20%: Niylah has appeared less in every season, and was not a major character to begin with. No offense, but I don't think they would waste screentime killing her character, but I won't rule anyone out.
Gaia: 10%: While Gaia is a larger character than Niylah, there's nobody at the moment who really wants her dead, although she could die in the conflict, so the only way she will die, is if Madi kills her at Sheidheda's command.
Jackson: 15%: Sachin Sahel is actually a pretty important member of the cast. He's too important to Miller, and not hated enough by Russel or Simone to die in the finale.
Miller: 15%: Same here. He's important to Jackson, and not hated that much by Russell or Simone. If he died, it will come as quite a shock.
Russell: 80%: He is evil. He should follow the same pattern as Dante, ALIE, McCreary, and Emerson: Death by Clarke. Since Clarke is Josephine, she might surprise him, and kill him in the finale.
Simone: 75%: Although Simone is more evil than Russell, she now has Abby as a host, so killing her might be hard for people like Clarke and Jackson, but she will most likely end up dead.
Indra: 50%: She has had less screen time this season, and now that Octavia can take care of and make decisions for hersel, she may not be needed as much on the show. Still, Russell and Simone are unpredictable, so I wouldn't put it past them to kill her.