The New England Patriots and the San Diego Chargers. The patriots are favourites now with Stallworth, Walker and Moss added to the wide receivers corp, and the signing of Adalius Thomas improves the defense. If they don't get to the superbowl, I think that the season should be considered a failure. San Diego had the best player last year in LaDanian Tomlinson, and they were downright amazing in the season, but they choked in the playoffs, and they got to prove that they have what it takes to win a superbowl, especially after changing coaches and not losing much in the off-season, they would have no more excuses.
As for which player have the most to prove, I think it would be Randy Moss. Did the patriots get the sulky, egoistic Randy Moss or did they get the Randy Moss that had the potential to be the best receiver in the NFL.
For coaches, I would say Jack Del Rio of the jacksonville jaguars. I respect him for wearing suits to games, which I think should be standard dressing for coaches, but he has not really improved the jaguars and they have a complicated qb situation that needs to be sorted out