Dany has good intentions but everything she touches turns to shit or dies.
Robert was a whoring drunk who spent the realm's money and had other people do all the work.
Renly liked the idea of being a king but not actually being one; namely, he thought being a king meant looking pretty and watching jousts.
Stannis is a hardass but he seems to take his responsibility seriously. Shame about the burning thing, although I think he might be coming around on that.
Balon's ambition was always too big for the Iron Islands.
Robb had good intentions and would have been OK in peacetime but he lost it on a few bad personal decisions.
We haven't really seen Aegon do any actual ruling, have we?
I'll throw in Joffrey -- an evil little sadist -- and Tommen -- who's still just a child.
Overall, despite his setbacks, I have to go with Stannis, too.
ETA: Another thing in Stannis' favor is, you can achieve a position of authority in his court without being a kissass or a born lordling. Would any of those other kings, even Robb, have elevated a Davos Seaworth type based on merit? I doubt it. Stannis is also one of the few people who actually would rather you didn't
blow sunshine up his ass.
Edited by Apple Martini, 24 February 2012 - 04:07 PM.