Lists for top QB's are getting harder and harder to make due to the changes in the game. The fact that you can't truly hit a QB anymore and the new wide receiver rules which went into effect in 2007 (I think) have made it far easier for QB's to pass today and it makes for much greater longevity. Who knows what Marino, Montana, Favre, Unitas, etc. would do if they were playing in today's league. Favre had his best year ever at age 39 on a new team with no good wide receivers, that should tell you a lot.
And I actually agree with BB about Favre, he'd often make you scream in joy and anger multiple times per game, but in the end his risk-taking and all the turnovers that came with it keep him from being near the very top despite all the volume stats. I would probably have him in the top 10 but not top 5. I think Brees is great but not underrated, for today's NFL he also turns it over too much and that's a big reason the Saints haven't been good in many years now. It may sound funny but in a way I miss watching Favre, a lot of my childhood memories are of watching #4 on Sundays and waiting all week for it. I'm also currently reading his biography, Gunslinger, and it is pretty awesome stuff.
Rodgers is sometimes almost too good and machine-like (aside from week 7 of last year to about week 10 of this year) that I take it for granted. It's not that I ever find him boring (far from it!) but it's a different feeling from Favre. And Favre was also the toughest SOB to ever play QB by a country mile and he is more of a natural leader. But with Rodgers there is way more consistency and he routinely makes some impossible plays where he buys himself close to 10 seconds and fires an off-balance, across his body pass that's right on the money. The one vs. Detroit last week was one of the greatest plays I've seen him make and it was pretty much a back-breaker in that game.
I am a little biased but I do think Rodgers is the best quarterback of all time but not the greatest. The greatest is either Montana or Brady because they have the rings, stats, and accolades. If Rodgers finishes within 1 ring of Brady and Montana I think he'd go down as the greatest ever because his stats blows away both of them in the regular and postseason and trust me, he isn't playing for Bill Belichick or Bill Walsh.
Rodgers playoff stats:
http://www.pro-football-reference.com/players/R/RodgAa00/gamelog/post/
Brady playoff stats:
http://www.pro-football-reference.com/players/B/BradTo00/gamelog/post/
Montana playoff stats:
http://www.pro-football-reference.com/players/M/MontJo01/gamelog/post/