If you're only refering to the Anakin of the prequels then I'd have to agree with you. But the Darth Vader of the OT (esp. ESB!) was perfectly capable of ruling the galaxy.
You see, they're two completely different
characters. Prequel Anakin was a friggin' idiot
that totally undermined the character of Darth Vader, IMHO.
Yet another reason why I despise the prequels with every fiber of my being.