Aside from the fact that he would have been the best king, he also had, in my opinion, the best claim. Cersei's children are illegitimate, so they are automatically ruled out. The Targaryens were defeated, and seeing as Robert was accepted as king, it is his line that should be counted, rather than the previous line, therefore ruling out Aegon and Dany. Stannis would have the best claim, were it not for the fact that he had been using dark magic, and had forsaken the gods of the Andals for R'hllor, and the King is supposed to be linked to the faith, however tenuously.

Robb was never after the iron throne. He wanted northern independence and justice for Ned's murder.

Renly was a big show pony with a lot of entitlement. I don't know how he would have been a good ruler unless the Tyrell family he married into were actually running the show, since they're clever if nothing else.

I hate to keep the Targargen legacy going but if it must be one of them, let it be Aegon. He's the heir, educated, has a lot of experience with the common folk.