There shouldn't be too many surprises here, except for the fact that George Scott didn't make the list (his salad years were with the Brewers, not Boston). The years listed are the years they played the position, not their total time in Boston.
5. Pete Runnels (1961-62): Runnels played five years in all with the Sox (1958-62) but spent the first three years at second base. When he moved to first, he continued his high level of play. In 1961 he batted .317 with an .810 OPS. In 1962 he made the All-Star team and was top 20 in the MVP voting. He batted .326 (leading the AL) with an .864 OPS and scored 80 runs while knocking in 60. After that season, Runnels was traded to the Houston Colt .45s*, spent a season-plus there and was released in early 1964. He passed away in 1991 at the age of 63 and was inducted into the Red Sox Hall of Fame in 2004.
4. Kevin Youkilis (2006-2010): Of all the Sox players I have watched, Youk is the only one I saw at every level of his development in the Boston organization (Lowell, Portland, The Bucket, Boston). And while you always knew he'd make the majors, I had no idea he would have been as good as he was, for as long as he was.
From 2006-10, Youk committed just 13 errors at first base. He held the all-time MLB record for most consecutive games at first without an error until Casey Kotchman broke it in 2010**. He won the Gold Glove for 1B in 2007 and went to the All-Star game twice at the position. His bat caught up to his glove in 2008 with a breakout year; 29 homers, 115 RBI, a .312 batting average and a .959 OPS, making him a contender for AL MVP*** and was in the top 10 for MVP voting in 2009. Over his seven years as the primary first baseman for Boston his average WAR (wins above replacement) was 4.25. For a three year period (2008-10) it was 6.1. At his peak he was one of the all-time best.
3. Mo Vaughan (1991-98): If you came of age in the 90s and were a Sox fan, the one constant on the team was Mo Vaughan. Between his debut in 1991 and his last year in Boston in 1998, the entire roster changed over except for Vaughan. The reason for that was simple; the guy could play.
Vaughan won the AL MVP award in 1995 and was Top 25 in voting six consecutive years (1993-98). Vaughan was a three time All-Star (1995-96, 98) and won the Silver Slugger award for 1B in 1995. From 1993-98 Vaughan never hit below .300, hit 26+ homers each year, averaged 110.5 RBI per season and always had an OPS of .915 or higher. He didn't have the best glove at first but was solid. Most importantly, he was beloved by Sox fans and still is to this day. Partially because of his running feud with Dan Shaughnessy but mostly because of the time he took to interact with fans and perform charity work on a level rarely seen in professional sports.
We all know it didn't go well for Mo after left Boston, but today he is continuing to do good work for the community. He owns OMNI New York LLC, a company that buys and rehabs housing in and around NYC, then turns it into low-cost housing. As great a player as Mo was, he is an even better man****.

