Stannis lost his battle already in Season 2. So he is out of the game for now.
Stannis is easily the best candidate for sitting on the iron throne.
Ned and Robb let their honor get in the way and which lead to bad decision after bad decision.
Eh, not really? Robb died because of a vow he broke. Hardly Eddard level of honour. Robb had no way of seeing what was coming, unlike Eddard. The entirety of Westeros is stunned by it.