I'd have things in three different categories
1. Brady - Won 5 Super Bowls (played in 7) and may soon be the stat king as well. Hard to argue against him as the best.
-------------------------------
2. Montana - A winner who led his team to 4 Super Bowls. Modest career stats among the best QBs, but 4 rings speak for themselves.
3. Manning - multiple Super Bowl winner and best career stats. Great professional. One of the rare QBs who was both great in college and the Pros.
--------------------------------
4. Elway - Won 2 Super Bowls (played in 5) and has a great stat resume. Gritty, passionate, great come from behind QB.
5. Rodgers - Super Bowl winner and owner of the best QB rating of all time, which counts for a lot
------------------
6. Brees - Super Bowl winner and one the best stat CVs in NFL history. Most accurate passer in NFL history and may end up the stat king before its all said and done. Has average 5k yards for the past 7 or so years.
7. Favre - Best gunslinger in NFL history; incredible stat CV and played in 2 SBs (winning 1). May be a couple of spots higher if not for the high INT numbers.
8. Young - Would easily be in the top 5 if he didn't spend his first two career years in the USFL, then Tampa Bay and the first half of his NFL career backing up Montana. One of the best rushing QBs among the greats
9. Marino - Best release in NFL history along with Fouts. Best QB to never win the big game otherwise he would be in the top 5. You can blame Montana for his lack of success
10. Unitas - Gotta have one of the oldies in the top 10 so United should be the one.
Plenty of others who could easily be considered for spots 6-10 including Graham, Aikman, Bradshaw, Eli Manning, Fouts, Rothlisberger, Kelly, Tarkenton etc.
(I'm probably making a glaring omission which will result in me having to rearrange everything but this is just off the top of my head).