Jim Thome to the Sox looks interesting...yeah, onto the real answer
If this is about which move has the biggest impact on a team, then the answer has to be the Marlins' Fire Sale part II. By "getting a payroll to reflect their revenues", the Marlins have taken a potential playoff contender and turned it into a team that will struggle to win 70 games. They have no second baseman, no center or left fielder, no catcher, and their starting rotation will consist of Dontrelle Willis and four scrubs. Maybe I'm proven wrong in 2009, but for the 2006 season, this has to have the biggest impact on any team.