Since the only thing I know about JR Ewing is that he was shot in a similar fashion to Mr Burns because of his cartoonish supervillianry, I admit this might not be the most enlightened answer. However, IMO Theo Epstein is probably the closest. Theo uses his financial might coupled with his intellectual superiority to bully other teams, ownership, etc etc into getting his way. And he's (generally) successful.
The only problem, of course, is that his title isn't "GM".