Honorable mention: Luis Quinones, Ival Goodman, Leo Durocher
5. Chuck Harmon (-0.1 WAR)
Harmon didn't play the longest, or the best. His 1955 was his best year, and he only OPS'd .734. But he's on this list because he's important. Why's that? Chuck Harmon was the first African-American player to play for the Reds. This, in a different era, in a city like Cincinnati. He's on the list.
4. Bip Roberts (4.8 WAR)
Bip was a solid player everywhere he went, but had his best year in Cincinnati. He made his only All-Star team in 1992, hitting .323/.393/.432 that year, with 44 stolen bases to boot. Roberts spent two years here, mostly in a utility role.
3. Eddie Taubensee (5.0 WAR)
Taubensee was the Reds catcher for the latter part of the 1990s, playing in almost 700 games total. He was always pretty good, but had his best year toward the end of his career in 1999. He hit .311/.354/.521 that year with a career high 22 homers.
2. Eddie Kasko (7.7 WAR)
Kasko was an infielder in the late 1950's, and played a major role on the pennant winning team in 1961. He made his only All-Star appearance that year, despite a .655 OPS. I guess he could field well.
1. Frank McCormick (27.4 WAR)
What a player Frank McCormick was. He broke in with the Reds in 1934, but didn't really get a regular role until 1938, where he was installed as the Reds' 1B. He made 7 straight All-Star teams with the Reds, and finished in the top 5 of the MVP voting 3 times.
He wore #10 until 1943, but played for the team until 1945. He led the league in hits 3 times, and had 128 RBI in 1939. (Trivia: how many hits did he have in his highest total season?)
McCormick also won the MVP in 1940. He hit .309/.367/.482 that year, which arguably isn't as good as his previous few seasons, but they won it all that year so I'm sure that helped.