The way he died and ruled the realm showed how stupid and unsuitable for a king he was. As for Ned I don't really think he was stupid, just honorable and unaware of evil people are capable of doing.

Ned wasnt stupid just Honorable to a Fault! Joffrey and Viserys are Retards!!!! Robert I think was just a Greedy Coward incapable of being king, Arys was Stupid too! Stannis is Awesome!

Hmm... Robert is here, because almost the whole bloody court knew or guessed that his kids were Jaime's except for him. Joffrey is Joffrey, Eddard darling thing, he ACTUALLY believed that after Robert's death people would do as their dead king bid in his final words written down by Eddard. Arys is Arys, Stannis believes this nonsense of Melisandre's about his being Azor Ahai, that's why he's here. Poor Viserys again thought that a man like Khal Drogo would do what he wanted. And yeah, poor Quentin has earned a place here, really.

Stannis and Eddard are stupid because of honor. Viserys and Robert because of alcohol. Arys Oakheart for both honor and love. I haven't gotten as far to know Quentyn's stupidity yet. Joffrey, however, was driven mad by power, he became a psychopath kid with too much power, and he is not a kid because he's 13, I am 13 too, but because he is childish and thinks he should have everything he wishes just because he's him, when he should not.

Well, Stannis... He believes (or just pretends his faith?) in the Azor Ahai thing because Melisandre´s fire magic really works somehow. She killed Renly, ser Cortnay, maybe Balon Greyjoy, Joffrey and Robb. She can see the future sometimes. If he wouldn´t accept the fire god, he would have been probably dead in the second or third book. He had no choice in this.
And what kind of honor allows him to murder his own brother? Look at them, all of them are dead and Stannis is still very much alive and in the Game. He has strange personality, but he is not stupid. Joffrey, Eddard and Viserys made the worst mistakes IMO.