I gotta put in my votes for the Mets.
With the exception of an extremely poor first half of the season, they finished strong enough to pull themselves into playoff contention, and they had one of the best offseasons in the majors in terms of trades and acquisitions.
Among their new personnel:
Mo Vaughan
Roberto Alomar
Roger Cedeno
Jeromy Burnitz
John Valentin
Plus, their pitching rotation is fairly respectable, with:
Leiter
Astacio
Estes
D'Amico
Trachsel
Their bullpen is stronger than it was last year, now boasting Chen, Guthrie, and Weathers, in addition to the NL's best closer in Benitez.
You don't get a much better infield than Vaughan, Alomar, Ordonez, and Alphonzo. Piazza behind the plate is without comparison in the majors in terms of offense. Their sole weak spot is centerfield, and Jay Payton is turning into a respectable hitter.
Leiter is a given that he's going to win 15+ games. Depending on how the other 4 pitch, this could be a team that could make a run in the playoffs if the wildcards on the team (Vaughan, Astacio, etc.) can stay healthy.