This was sort of a tough call. Not because there were a lot of other options. Most designated hitters in Red Sox history were big stars at some point in their careers, so it was tough to be underrated. No, the reason this was a tough call was because 1996 was clearly Reggie Jefferson's best season, but he played nearly as many games in left field as at designated hitter, and he played some first base as well. In fact, the only reason Jefferson played DH at all in 1996 was due to an injury to starter Jose Canseco.
Reggie Jefferson was originally drafted by the Reds and bounced from Cincinnati to Cleveland to Seattle in his Major League career before landing with the Red Sox in 1995. In those seasons prior to joining Boston, he only played in more than 65 games once. He typically hit for decent batting averages though and Boston took a flyer on him as a free agent in April of 1995. That season, he was decent in short work.
1996 saw Jefferson step in at a time when the team needed help and finally succeed, allowing him to keep a Major League job for a few years. He played in a then-career high 122 games. He hit a shocking .347/.388/.593 with 19 home runs, 30 doubles, and driving in 74 runs. His home runs, RBIs, triples, and slash line would all remain career highs.
Jefferson was part of a platoon at DH for most of 1997, until Mike Stanley was traded to the Yankees. For a time he was even contending for the batting title. He continued to be productive for Boston through the 1998 season before declining in 1999. That was his final season in the Majors. Jefferson finished his career with a .300/.349/.474 line, an impressive line for a player who never played in an All Star Game.