Second base is unique among positions in that no single Diamondback has multiple seasons in the top-five by fWAR; in fact, the top seven seasons at second base (minimum 200 plate appearances, as in other articles in this series) are all unique. Aaron Hill's 2013 season, in 8th place, is the first repeat. So the seasons that follow are the top-five seasons at second base, by fWAR, in Diamondbacks history.
Jay Bell, 1999
One of the first high-profile free agents signed by the Diamondbacks (in November 1997) Bell played primarily shortstop in 1998 (as he had throughout his career) before moving to second base in 1999. He had a storybook season, to put it mildly. Not only were his 38 home runs the most by a second baseman in team history, but one of them won a fan a million dollars. Hard to top that storyline. He posted the highest ISO and third highest wRC by any Diamondbacks second baseman, while posting an OPS+ of 131 (a career high) and playing average defense. Bell would become the only second baseman in team history to be voted to the All Star team (along with Matt Williams; an attempt to write-in Luis Gonzalez fell well short.)
Craig Counsell, 2005
Of the players on this list, Counsell is the only one that had a negative offensive value, but his defense was good enough to make up for it. Counsell had the most defensive value by any full-time second baseman on the team in the UZR era. What elevates his 2005 season into the top-five overall was posting career-highs in doubles and home runs. He hit 9 home runs, so not exactly a power surge. He also improved his OBP, and the combination of the two gave him an OPS+ of 89. He was worth 2.8 fWAR. But while Fangraphs liked his defense, Baseball-Reference loved it. So much so that he was worth 5.5 bWAR in 2005. Counsell may have been the best all-around second baseman in franchise history (since Roberto Alomar was merely a shadow of himself when he was with the team in 2004) and 2005 was his best season.
Aaron Hill, 2012
When the Diamondbacks traded for Hill in 2011, it was for a middle infielder who hadn't shown much with the bat in several years, and with a glove that was above-average at second and poor at shortstop. He showed promise with the bat in 2011, and completely went off in 2012, posting career highs in all the rate stats and most of the counting stats. He hit over .300 for the only time in his career (.302) while posting the highest OPS+/wRC by a second baseman in franchise history. While not noted for his speed, he stole 14 bases. He was worth 5.3 fWAR and 5.0 bWAR. He won the Silver Slugger and received an MVP vote. Had the 2015 Diamondbacks gotten 2012-level production out of Hill, it's not a stretch to say they would have been in contention up until the last week or two of the season.
Kelly Johnson, 2010
And now we have the player traded for Aaron Hill (and John McDonald). Johnson was regarded as average defensively by B-Ref, but as above-average by Fangraphs. But it was his offensive explosion in 2010 that gave him the most valuable season at second base in franchise history, according to Fangraphs. His 5.4 fWAR narrowly edges Hill, and despite not valuing his defense extremely highly, his 4.7 bWAR is pretty good, too. Johnson posted career highs in almost every offensive category. Nor was his BABIP the cause; his .339 BABIP was a bit high, but not the highest on this list. His offensive production was almost identical to what Hill would post two years later, and his defense was better. This was the classic example of a one-year wonder.
Junior Spivey, 2002
Spivey joins Bell as the only players on this list to make the All Star team, as he was selected by Bob Brenly. He had a career year, posting his highest offensive numbers across the board. He posted 4.3 fWAR and 3.9 bWAR, along with hitting .301, knocking 16 home runs, stealing 11 bases, and posting a wRC of 124. His defense was questionable, but he fit well in the offense. Unfortunately, the only time he showed anything close to this level of offense again would be after he was dealt to Milwaukee as part of the ill-fated Richie Sexson trade.