From the deleted scene from Nemesis it was a Commander named Martin Madden played by Steven Culp who later went on to play Major Hayes in Enterprise. You may remember him as the CIA Agent Clayton Webb in the series JAG.
In the books it was Worf.
Rick Berman in the Nemsis commentary said that he decided that Picard's new first officer should be one of the TNG crewmembers and had the scene cut. Admiral William Thomas Riker 21:21, July 26, 2011 (UTC)