Final answer:
In Season 3, Episode 7 "Branch Closing" of The Office, Jan's first choice to run Dunder Mifflin Northeast was Josh Porter, before he decided to leave for a position at Staples.
Step-by-step explanation:
In the episode "Branch Closing" of The Office, which is Season 3, Episode 7, Jan's first choice for running Dunder Mifflin Northeast was Josh Porter, who was the Regional Manager of the Stamford branch. This decision was disclosed during the episode when Jan and Jim have a conversation about the fate of the Scranton branch which is in jeopardy of closing. Jim is told by Jan that Josh is being considered for a promotion to oversee the newly consolidated Northeast branches. However, Josh leverages this offer with Staples to get a senior management position there, and consequently Decides to resign from Dunder Mifflin.