Wait, he was a Red Sox third baseman?
Adrian Beltre (2010)
This selection is the closest I straddle the line of "I forgot he played for the Sox" and "Who doesn't remember him in a Red Sox uniform", but in the grand scheme of things, Adrian Beltre is not going to be wearing a Red Sox hat when he gets inducted into Cooperstown. Beltre was one of the best third basemen of his generation, and I will always remember being upset when the Red Sox let Beltre walk in free agency after the 2010 season.
Beltre raked in 2010 for the Red Sox, leading the league in doubles (49), while ending the season with a .321 batting average, an OPS of .919, and an OPS+ of 141, leading him to a 9th place finish in MVP voting that season. Beltre signed with the Rangers after the season, ending his Red Sox stint far too soon, and making the Red Sox stand out like a sore thumb on his Baseball Reference page.
Honorable Mentions: Much like catcher and first base, third basemen in Red Sox history are a lot of guys who stuck around in Boston for a little while, if not always playing at third base during their tenures. The next best choice I had was Pinky Higgins, a solid third baseman who played on the 1946 AL pennant-winning team, but Beltre is the clear choice here.