Many people assessing on win records here, and winning records are a very incomplete way of assessing a QB's value or skill. In talent, records, skills, historical record, past performance, and overall ability, the only active QB that measures up with Brady is Favre.
So, my question is, is this "Best This Year" or "Best Overall"? BIG difference. Whatever Manning has done, he has not won as many Super Bowls as Brady, nor have any of the others on this list or any currently playing in the NFL ... also, much of Manning's success has come from the quality of his offensive line.
The guy has forever to throw the ball, while Brady, especially this year, is constantly under pressure in the pocket. Of all the QB's playing in the NFL today, Tom Brady is the most successful, the most winning, and the best under pressure.
So again, are you talking about the QB with the best performance this year, or the best active QB period? The answer to the latter is Tom Brady.