#42 is a tough one, because despite it being retired for Jackie Robinson, there are an absolute ton of guys who wore it briefly for the Reds.
5. Santo Alcala (-0.8 WAR)
Alcala is one of those names that I recognize for being on the Big Red Machine team, but couldn't pick out of a lineup of one. The Dominican pitcher went 11-4 in 21 starts in his rookie season. That was his best season, as he was traded to the Expos in the middle of the '77 season, which was his last in the big leagues.
4. Tommy de la Cruz (2.3 WAR)
Tommy de la Cruz is an interesting guy to me. A longtime player in the Cuban League, he got an opportunity to play in MLB in World War II. He pitched for the Reds for only one year, in 1944, and went 9-9 with a 3.25 ERA in 34 games (20 of them being starts). He threw the first one-hitter by a Latin American player, which I bet you didn't know was a record that belonged to a Red.
His story gets even more interesting, though. After his debut big league season at age 32, he got drafted into the U.S. Army to go to war. He opted to enlist in the Cuban Army instead, and as a result had to leave the U.S. He went to Mexico and pitched in that league for 3 seasons, but was never permitted back into MLB.
3. Rick Mahler (2.3 WAR)
Mahler has a ring too, as he pitched for the Reds from 1989 to 1990. He started his career pitching for Atlanta, and did for 10 years before moving to Cincinnati. He was mostly a starter in '89, but pitched primarily out of the bullpen in the Reds' World Series year. His 242 hits given up was the most in the league in '89... and that was the 4th time he managed that feat. So, he was ok.
2. Billy McCool (6.0 WAR)
In addition to having an awesome name, McCool was a pretty good reliever for the Reds from 1964 to 1968. He made the All-Star team in 1966, when he appeared in 57 games with a 2.48 ERA. He had 18 saves, too. The Reds lost him in the expansion draft. Trivia: to what team?
1. Jim Brosnan (6.5 WAR)
Brosnan was another Reds reliever, pitching from 1959 to 1963. He was mostly the closer for those teams, including the '61 World Series team. That year, he went 10-4 in relief with a 3.04 ERA and 16 saves.
Even better... he's a Cincinnati guy. He went to Elder, was born here, and is still alive. He's also known as a little bit of a writer, as he was one of the first players to write a memoir about a season. His "The Long Season" was about the '59 Reds, and helped pave the way for the excellent "Ball Four". He wrote a book about the '61 Reds as well, and went on to be a sportscaster.