Through 2019. Ranked by WAR (baseball-reference+Clay Davenport, the best WAR number that exists).
1. 1965 Willie Mays 10.95
2. 1964 Willie Mays 10.95
3. 1963 Willie Mays 10.5
4. 1962 Willie Mays 9.9
5. 1954 Willie Mays 9.9
6. 1958 Willie Mays 9.6
7. 1955 Willie Mays 9.2
8. 1960 Willie Mays 8.5
9. 1957 Willie Mays 8.45
10. 1966 Willie Mays 8.4
10. 1966 Willie Mays 8.4
11. 1961 Willie Mays 8.25
12. 1956 Willie Mays 7.35
13. 1988 Brett Butler 6.95
14. 1959 Willie Mays 6.95
15. 1968 Willie Mays 6.45
16. 1971 Willie Mays 6.1
17. 1949 Bobby Thomson 5.9
18. 2010 Andres Torres 5.7
19. 1905 Mike Donlin 5.55
20. 1990 Brett Butler 5.2
19. 1905 Mike Donlin 5.55
20. 1990 Brett Butler 5.2
21. 2008 Randy Winn 5.1
22. 2012 Angel Pagan 5
23. 1984 Chili Davis 4.85
24. 1970 Willie Mays 4.6
25. 1896 George Van Haltren 4.6
26. 1935 Hank Leiber 4.35
27. 1967 Willie Mays 4.2
27. 1967 Willie Mays 4.2
28. 1910 Fred Snodgrass 4.1
29. 1903 Roger Bresnahan 4.1
30. 1989 Brett Butler 3.75
30. 1989 Brett Butler 3.75
31. 1973 Garry Maddox 3.65
32. 1907 Cy Seymour 3.65
33. 1898 George Van Haltren 3.65
34. 1951 Willie Mays 3.6
35. 2005 Randy Winn 3.5
36. 1984 Dan Gladden 3.45
37. 1900 George Van Haltren 3.45
38. 1952 Hank Thompson 3.35
39. 1889 George Gore 3.35
39. 1889 George Gore 3.35
40. 1986 Dan Gladden 3.2
41. 1991 Willie McGee 3.15
42. 1917 Benny Kauff 3.15
43. 1897 George Van Haltren 3.15
44. 1904 Roger Bresnahan 3.05
44. 1904 Roger Bresnahan 3.05
45. 1947 Bobby Thomson 2.95
46. 1979 Billy North 2.9
47. 1911 Fred Snodgrass 2.9
47. 1911 Fred Snodgrass 2.9
48. 1916 Benny Kauff 2.65
49. 1946 Goody Rosen 2.6
50. 1975 Von Joshua 2.55
50. 1975 Von Joshua 2.55
And the worst center field season in Giants history 1944 -3.95 Johnny Rucker
