Quarterback is the most important position in professional football, but every quarterback is dependent on his team context to win. It is fashionable to cite a quarterback's record as a starter (I'm sure I've done it). But this assigns credit or blame to the quarterback for the entire team context.
If we're going to look to the team's performance in assessing quarterbacks, let's at least cut the team context in in half and look at the offensive performance alone. Certainly that is also dependent on team context, but it's not dependent on the performance of the defense, which the quarterback has no control over at all. Instead of citing a quarterback's winning percentage, cite his team offensive ranking.
For example, under Peyton Manning, the Colts have ranked in the top-five in both points and offensive yards in eight of 10 seasons. The Colts have won a lot of games during that time. The defense has been up and down for the last decade, but that really shouldn't matter to an assessment of Peyton Manning. He's made the Colts' offense one of the best in the league for years. Certainly he's been helped by the likes of Marvin Harrison, Reggie Wayne, Edgerrin James, and Joseph Addai: there's still context. But we can at least cut that context by half (or more, including special teams) and look at what the quarterback actually can control. Compare Manning to Tom Brady, who has a better winning percentage. In Brady's seven years as starter, the Patriots ranked in the top-five in scoring twice and offensive yards once.
Examining a QB's team offensive ranking rather than his team record might allow us to re-evaluate some players. For example, under Trent Green, the Chiefs led the league in points in 2002 and 2003, and in offensive yards in 2004 and 2005. Certainly a great offensive line and a great running attack are largely responsible for that. But do you think of Trent Green as an elite quarterback at all? Me neither. But during that time, the Chiefs' defense ranked 32nd, 29th, 31st, and 25th in yards allowed. If the Chiefs defense had ranked in the top-10, would the Chiefs have won a Super Bowl? Maybe. And then might you think of Trent Green as an elite quarterback? He could at least be considered a Troy Aikman type of QB.
Looking at how a quarterback helps his team succeed is important. But looking at a win-loss record, and crediting that to the quarterback, pretends the quarterback has control over defense and special teams. Looking at a team's offensive rankings actually shows how the quarterback was able to help his team in the area he can control.