How great was Gonzo in 2001? Think of all the great players ever to play left field. This list includes Babe Ruth, Barry Bonds, Ted Williams, Rickey Henderson, and Carl Yastrzremski. Luis Gonzalez and his 8.9 fWAR in 2001 rank him in a tie for the 32nd best season in left field in baseball history. Expand it to all outfield positions, and it's tied for 69th. Basically, Gonzalez in 2001 was historic, even though he was overshadowed by Barry Bonds and his 73 home runs. With 57 home runs, Gonzalez had the fourth most by a left fielder all-time, trailing only Bonds in 2001 and Babe Ruth twice, in 1927 and 1921. But for Gonzo, it wasn't just one season, as the top-five seasons by a left fielder in franchise history all belong to Gonzo.
Once again, let's look at the backup to Gonzo. The same rules apply here as did at shortstop: in order to qualify, a player must have at least 300 plate appearances and have played more at left field than at any other position. That means that Ender Inciarte, who put together solid numbers both seasons with the team and would appear on the ballot without that last qualification, gets left off. He spent more time at center field in 2014 thanks to Pollock's injury, and more time in right field last year.
Eric Byrnes, 2007
Byrnes epitomized the Diamondbacks in 2007, in that while he didn't put up MVP numbers (.286/.353/.460 for 3.7 bWAR and 3.6 fWAR) he still managed to get some votes. That was largely because he was, in a sense, the face of a team that gave up more runs than they scored but still managed to post the best record in the National League. Byrnes wasn't too bad defensively in 2007, though, rating as average by UZR and as very good by DRS. He stole 50 bases, good for second-most in franchise history (behind Tony Womack's 72 in 1999.) He followed up a decent season by hitting a home run and driving in 3 in the NLDS against the Cubs, but after doubling in Game 1 of the NLCS to give the D-backs an early lead, he went 2-for-16 the rest of the way as the Diamondbacks wouldn't take the lead again until briefly in Game 4.
Conor Jackson, 2008
I can copy and paste a lot of what I put down for Jackson on the first base article. At 26, he had a breakout season in 2008, hitting .300, slugging .446, and playing solid defense with good baserunning. He looked set to develop into a solid 3 win player. Season highlights for Jackson included an opportunity to hit for the cycle, finishing a double short (having tripled in this last at-bat.) Jackson didn't strike out much, only in 10% of plate appearances. Sure, he could have stood to walk more (only 9.6% of plate appearances) but he was a tough out every time he was at the plate, showed power, and even showed some speed (10 stolen bases.) He also won NL player of the week twice in 2008. Sadly, his career was derailed with his bout with Valley Fever starting in 2009, and he would never be the same player again.
Gerardo Parra, 2011
The legend of Gerardo Parra might have been born earlier, but it was in 2011 when he showed the potential to be one of the better outfielders in the NL. First, his defensive prowess was recognized with a Gold Glove, one of two he would win with the Diamondbacks. But he also put together a career year offensively, slashing .292/.357/.427, for an OPS+ of 113. He stole 15 bases, and was caught only once, setting a franchise high for stolen base percentage. Parra was a valuable member of a team that surprised by winning the NL West. Unfortunately, his 1-for-18 performance in the NLDS certainly didn't help the cause, but as far as the regular season was concerned, Parra was excellent. He was worth 3 bWAR and 2.7 fWAR.
David Peralta, 2015
Count me among the many that had low expectations for Peralta in 2015, after he cooled off following an insane start to his career in 2014. But he got substantially better instead. For starters, he slashed .312/.371/.522 (easily the best offensive numbers for a left fielder since Gonzo) for a wRC+ of 138, which trails only Gonzo in 2001. It was arguably the second-best offensive season by a Diamondbacks left fielder of all time. He also hit 17 home runs, led the league in triples with 10, and showed his willingness to play all three outfield positions as required. His defense was rated as below-average by both UZR and DRS, but it was still plenty acceptable, and he posted 3.7 bWAR and fWAR.
Cody Ross, 2013
Our memories of Ross are far from fond, both because of his failure to live up to either hype or contract and his poor attitude towards criticism. But his 2013 wasn't nearly as bad as we thought, looking back at it. Sure, his defense when he had to play right field was quite poor, but he was generally in left, where he was average. And despite struggling early, his offense had rebounded; after struggling for about two months, he slashed .289/.350/.456 from June 1 until his season ended with horrific injury. Had he continued that pace, he would have had a career year offensively, even with his slow start. As it was, he posted an OPS+ of 104, and although his splits were extreme, he provided positive value. He was worth 2.5 bWAR and 1.9 fWAR.