To figure out who has the best home-field advantage in football, I went through the recent past and figured out the point differential for each team in their regular-season home and road games.1 To figure out their home-field advantage, I subtracted the road differential from the home differential and divided by two (since we're measuring home-field performance versus, theoretically, a neutral field). Games that weren't played at a team's typical home stadium, like the London international series or Buffalo's annual game in Toronto, were considered to be neutral-site games and excluded from the study.
As an example, take the home-field performance of the Seahawks since 2002, when they opened their new stadium in downtown Seattle. The average Seahawks home game over that time frame has seen them win by exactly one touchdown, 7.0 points. Only four other teams have won their home games by a larger average margin than the Seahawks. Those teams are each better on the road, though, than Seattle; the Seahawks lose their average road game by 3.3 points. That's a swing of 10.3 points the moment Seattle gets on a plane, which suggests that Seattle's home-field advantage is worth 5.2 points. That is, as you might expect, the most impressive observed home-field advantage in football during that time frame.
Here's a table with each team's home-field advantage in its current stadium, leaving out any games from before it moved into its new stadium. This only includes games since 2002, since that's when the NFL expanded to its current 32-team size, divisional structure, and schedule.
If you don't believe point differential really captures the way a team plays at home and on the road, there's an even simpler way to analyze this: winning percentage at home and on the road. I'm a little uncomfortable with the small samples2 for teams with brand-new stadiums, but the numbers are mostly similar to the table above. Here are the top and bottom five teams in terms of the gap in their winning percentage at home and on the road:
I do realize a lof of this has to do with just having an overall shitty teams since 2002.