The Champion is a cop-out answer, because WoW has never been a game where you can permanently die as a PC.
I'd actually say Nathanos myself, out of those.
Anduin hasn't really been in that many situations where he probably should have died, and yet Deus Ex Machina and/or Plot Armour (i.e. NPCs making completely irrational decisions that stop him dying) have saved him. I mean, he's been in a few, but I'm not sure it's any more than a bunch of random NPCs who are still alive. Until he's been in serious danger a lot more often he's not going to be really justifiable.
Sylvanas you can make a better case for, but there are generally specific, lore-justifiable, and sense-making reasons for her survival, rather than just Deus Ex Machinas or irrational NPC decisions. Plus she's actually died what, at least twice? That's pretty flimsy plot armour. She's come back for specific reasons. She also has pretty well-established massive powers which are going to make her a lot harder to properly kill than other people. If we actually look as Deus Ex Machinas and so on, I suspect Thrall is a much worse offender than she is. But he's well-liked, and Sylvanas is fucking jerk, so people ignore that and vote against the jerk. Which is understandable but I think inaccurate.
Whereas Nathanos keeps getting into ridiculous, incredibly deadly situations, where the smart, reasonable thing to do, would be for people to kill him. Except they don't, or they were going to be some kind of unlikely event or Deus Ex Machina saves him. And it keeps happening. And he does NOT have massive magical powers that would make him all that hard to kill. So to me, relatively speaking, his Plot Armour is much thicker than Anduin or Sylv. Though maybe his run will end soon.