Going a bit old school today, as there really haven't been too many #51s of late. Let's go.
5. Tom Carroll (-1.2 WAR)
If you're the only guy with a ring with a certain number, you get special treatment. Tom Carroll won his as a long reliever with the Big Red Machine, getting 7 starts in that season as well. Numbers aren't too kind to Carroll, who didn't strike a lot of guys out and gave up a lot of hits, but he kept the ball in the park and ended '75 with a 4.98 ERA.
4. Mike LaCoss (-0.6 WAR)
The former 3rd round pick was a fixture on the Reds roatations of the late 70's and early 80's. His WAR is skewed by the 3 seasons where he wasn't very good, but in 1979, LaCoss went 14-8 with a 3.50 ERA, and was arguably the #2 starter at age 23 (behind a pitcher you may have heard of named Tom Seaver). That team won the NL West, but got swept in the NLCS by Pittsburgh, who of course went on to win it all.
3. Sean Marshall (1.6 WAR)
The Marshall Plan was looking pretty successful in 2012, which is when Marshall wore this number (he has since switched to #45, if you're keeping track in your programs). He put up a 2.51 ERA in 73 appearances as a setup man/LOOGY/closer hybrid for a division champion.
2. Jared Burton (2.5 WAR)
I don't think the average Reds fan appreciates how good Burton was when he was with the Reds. A former Rule 5 pick (trivia question: from where?), Burton put up a 2.51 ERA in his rookie season in 2007, his best with the Reds. He'd go on to have injury problems, but spent 5 seasons with the Reds, before moving to his current home in Minnesota.
1. Art Fowler (6.4 WAR)
Spanning all kinds of different decades with these picks, aren't we? Fowler is really the only choice, with how good he was in his time with the RedLEGS from 1954 to 1957. In those 4 seasons, he put up a 37-31 record in 87 starts, and put up a 3.83 ERA as a 31-year-old rookie (!) in '54. That's right, he spent 10 years in the minor leagues, but pitched until he was 41, ending his career with the Angels.
He's probably best known for being a long-time pitching coach in the big leagues, managing under his friend Billy Martin. Fowler coached for 5 teams, including many stints with the Yankees (corresponding to Martin's, of course). Fowler passed away in 2005 in his native Spartanburg, SC.