My MVP list has been all over the place: I drop a player one week and add him back the next week. I'm not remotely confident in this list: next week Brees could throw three interceptions or Eli Manning could finish off the Cowboys on national TV. This is simply what it looks like to me right now.
Brees leads the league in passing yards and passing touchdowns. If the Saints make the playoffs, it will be because Brees deserves MVP; if they miss the playoffs, it will be because he doesn't. I've addressed this.
Statistically, Manning has been much better in non-MVP years (of course, I think he deserved MVP in some of those years, particularly 2005). His yards per attempt are the lowest since his rookie season. But context matters: the MVP field is more open this season, and Manning has led this Colts' team to a lot of late-game wins.
Peterson is the NFL's leading rusher (1,413 yards) for an 8-5 division leader. He's had eight 100 yard games, and in major division games he rushed for 192 yards against the Packers and 131 yards against the Bears. A rushing title plus a division title would make Peterson a strong contender.
Troy Polamalu/James Harrison
The Pittsburgh Steelers rank #1 in points allowed and #1 in yards allowed. Despite an inconsistent and mediocre offense, the Steelers are 10-3. Polamalu has seven interceptions and 16 pass deflections. Harrison has 15 sacks, seven forced fumbles, one interception, and one safety. The problem is I don't know which player is actually more valuable: both players have had monster impacts.