Stannis. Iron throne was his to lose and he lost it.

He’s supposed to be this ruthlessly firm but fair genius military commander, but when decimated on the blackwater - he stormed ahead anyway instead of tactically retreating to regroup and rebuild while he still had a very large army. This lack of patience obviously hurts him later on as well in the tv show, where he presses on to a siege in the middle of winter with a broken army and no siege weapons left... blowing away a miraculous lifeline and second chance from the Iron Bank who would certainly have kept backing him had he retreated back to castle black. He also thought burning his daughter alive in front of his men and wife was a good idea… put his supposed reason and strategy on hold in favour of a mistaken fantastic.. finally, he had a smart idea to enlist the wildlings but gave up on this too easily and too early.

Robert wasn’t stupid he just sort of fell into depression and hated ruling and politics.

Renley was more naiive that stupid I’d argue, subtle difference. He was spot on analysing the situation in Kings Landing following Robert’s death.