First, there is no such thing as a flawless QB, even Brady has faults. And while there are and have been some amazing offense units in league history, no team is unstoppable. But lets just think about it for fun. Joe Montana went to the Chiefs, it definitely improved the team, but weren't unstoppable. Manning went to Denver, it definitely helped the team, 2 SB appearances, 1 win, but they were clearly stoppable in the SB against Seattle, which by the way Denver was the #1 offense in the league at that point. But to answer your question, a good QB can always improve a team, but no one is unstoppable.