I decided to be as objective as possible here, rather than picking anti-favorites, so I went by the player posting the worst bWAR at each position. For position players, I went with a minimum of 100 games played for the Diamondbacks, and at least half of them at the spot in question. This helps eliminate those who were fleeting meteors of suck across the firmament of Bank One Ballpark or Chase Field, in favor of those who had us throwing things at the television for longer spells. Rather than just sticking to nine, I also tossed in a relief pitcher and a utility player, who didn't reach the 50% threshold at any one spot, but still delivered adequately below replacement level.
C. Rod Barajas: -2.0 WAR, 211 games
Easily the worst position player in Diamondbacks history, we had him at the beginning of his career, when he was raw and still clearly learning his craft. Didn't stop him from winning a World Series ring with us in 2011 (homering in Game 5), and he went on to play for almost a decade after we released him. Barajas was surprisingly non-sucky, putting up 6.8 WAR the rest of the way, and did very nicely, earning almost $20 million before retiring.
1B. Shea Hillenbrand: 0.4 WAR, 233 games
Dealt from the Red Sox in exchange for Byung-Hyun Kim, this ended up being a lose-lose trade for both teams. Hillenbrand started off at third, but moved to first. While his bat was okay - a 100 OPS+ in his time with Arizona - he had true hands of stone, committing 28 errors here. That did a pretty good job of negating any value provided by his offense. We traded him to Toronto, and he was an All-Star for them in 2005.
2B. Matt Kata: 0.9 WAR, 150 games
Kata wasn't actually too bad. He was just your prototypical light-hitting middle-infielder (OPS+ 77 with us), who had a decent enough glove and was paid major-league minimum for his time here. I feel kinda guilty about including him, but there's no other qualifying player at the position worth less than five wins. Sorry, Matt.
SS. Royce Clayton: 0.1 WAR, 143 games
There were some other candidates here: Willie Bloomquist (0.3 in 225 games) or even Tony Womack (1.3 in 629), but Royce's barely replacement-level tenure seems worthy of note. For some reason, I keep confusing Royce with Damion Easley, who at least did hit three homers in a single game once. Clayton never did anything as significant. In fact, I'm not sure I can remember anything of note about his time in an Arizona uniform.
3B. Eric Chavez: 0.6 WAR, 124 games
This is another position where the "winner" was less bad at his position, more the least good: even Troy Glaus, the runner-up, posted 3.4 WAR in his single year here. Chavez just scraped in over the 50% mark too, with 63 games at third. He was signed mostly as a power-bat off the bench, and delivered adequately there, with a 120 OPS+ over his two seasons. However, he was seven years removed from his last Gold Glove and that showed in his defense.
LF. Jason Kubel: 0.4 WAR, 230 games
If we lumped the outfield together, Mark Trumbo's -0.6 WAR so far would merit inclusion. However, Trumbo has played more at 1B than either outfield position - though not enough to qualify at first - so dodges both bullets, Kubel's first year here was okay, but his health and value tanked in 2013, and he was dreadful. Indeed, fWAR has him as tied for the worst season ever by a D-back, despite only 89 games. While bWAR is kinder, it still ain't good.
CF. Luis Terrero: -0.5 WAR, 155 games
Three words sum up Terrero's time with us. "Hidden ball trick." For in August 2005, he became one of the rare victims of this in the modern era, straying off third in the belief the pitcher had the ball, when actually it was still in the hands of Marlins' third-baseman, Mike Lowell. I don't know if that is figured into his WAR - probably not - but he probably deserves inclusion for that alone. So, I'll just leave this here...
RF. Karim Garcia, -1.5 WAR, 113 games
Our worst-ever outfielder, certainly played a part in why our franchise's debut season was so forgettable. With a 66 OPS+ and negative defensive value, it was a miracle that at the end of the year we were able to get anything at all for him - even a journeyman 31-year-old, who had never hit more than 23 home-runs in a season... Less than 3 years later, the player we received plus cash for Garcia, had the winning hit in Game 7 of the World Series...
Utility. Andy Green, -1,2 WAR, 136 games
There is just one Diamondback to have appeared in at least 100 games for the team and hit below the Uecker Line - at least for now, with Nick Ahmed on notice. For Green was Ahmed before Ahmed was, owning a triple-slash as a Diamondback of .199/.280/.265, playing in a much more offensively-minded era from 2004-06. A subsequent career path shows, the baseball smarts of our current third-base coach clearly exceeded his offensive skills.
SP. Russ Ortiz, -2.2 WAR, 28 starts
This is my unsurprised face at the result here. $33 million clearly doesn't get you what it used to - that being what we paid for four years of Ortiz's service in December 2004. Barely eighteen months later, we realized what a bad idea this was, and released him. It resulted in Arizona eating the $22 million remaining, which makes the dumping of Trevor Cahill and Cody Ross look like pocket change.
RP. Eddie Oropesa, -1.9 WAR, 79 appearances
It's amazing a pitcher could play as often, with a 7.59 ERA. Indeed, he may be the worst reliever in major-league history: Oropesa's career figure (7.34) is the highest by any pitcher with 100+ games. His 2002 season for us was particularly brutal, one of only two players to appear 30+ times in a year and have an ERA above ten (10.30). The other was also partly in Arizona - Javier Lopez in 2005 - but Oropesa blew chunks harder, for longer.