Smartest is between Barzini and Roth.
Both played it out beautifully, but I think Barzini had more muscle than Roth. He could have out numbered hit with Buttons and plus Roth wasn't liked by the government.
Barzini had politicians in his pockets like Vito.

Roth was running away in the end, he didn't seem to be smart when it came to legal issues.
If Barzini lasted the 'steps' killing, he could have outfought Michael and the 5 families were all allies to him in order to prevent a long war.

BARZINI! IS! THE! VILLANEOUS!