There's very little doubt that Jim Kaat was the best fielding pitcher in the American League from 1962 through 1965. At the conclusion of each of those four seasons, Kaat was fittingly honored with four consecutive Gold Glove Awards.
The key defensive metric - Range Factor - was significantly better than what was "league average" over that four season span (as shown below):
Kaat Lg Avg
1962 - 2.94 - 2.08
1963 - 3.13 - 1.99
1964 - 2.37 - 1.90
1965 - 2.69 - 2.04
For some reason, in 1966, Kaat's fielding prowess diminished significantly. He was now just an average fielding pitcher, at best; although apparently, no one really noticed. As he aged, his skills diminished further, and by the early '70's, Kitty Kaat was significantly below average with the glove, as opposed to the guys who should have won the Gold Glove (as shown below):
Kaat Lg Avg Revised GG Award Winner (Range Factor)
1966 - 1.92 - 1.99 Joe Horlen (3.50)
1967 - 2.02 - 1.96 Tommy John (3.63) (Also won an imaginary '73 GG in the NL)
1968 - 1.77 - 1.89 Tommy John (3.25)
1969 - 1.41 - 1.88 Mel Stottlemyre (3.33)
1970 - 2.27 - 1.86 Joe Horlen (3.34)
1971 - 1.87 - 1.98 Mike Hedlund (2.76)
1972 - 1.91 - 1.86 Mel Stottlemyre (2.42)
1973 - 1.44 - 1.86 Mel Stottlemyre (2.47)
1974 - 1.53 - 1.81 Steve Busby (2.37)
1975 - 1.60 - 1.87 Steve Busby (2.56)
Yet, Jim Kaat still managed to win a Gold Glove Award, year in and year out.
In 1976, Kaat moved to the National League, winning his last two Gold Glove Awards - bringing his grand total to sixteen consecutive - a record for pitchers; equaling the major league record for most consecutive Gold Gloves won (Brooks Robinson - the greatest defensive third baseman of all-time also won 16 in a row):
Kaat Lg Avg
1976 - 1.46 - 1.88 Rick Reuschel (2.63) (Revised GG Winner)
1977 - 1.46 - 1.85 Rick Reuschel (2.57)
More often than not, the managers and coaches polled for this prestigious award get it right. They got it right the first four times with the affable Kaat. Somehow, they failed to notice the regression over those final twelve seasons. Consequently, they failed to notice the fielding prowess of Horlen (twice), John (twice), Stottlemyre (thrice), Hedlund (once), Busby (twice) and Reuschel (twice).
Kaat is a borderline Hall of Famer, who just missed getting voted in by the latest Veteran's Committee panel (needed 12 votes - got 10). Certainly, those in favor of his induction cite the 287 career wins he amassed in 25 major league seasons; not to mention his impressive collection of Rawlings' Gold Glove Awards - sixteen to be exact. Whereas this achievement earned Kaat a place in Rawlings' Hall of Fame in 1991, that grand total needs to be taken with a grain of salt when considering his true Hall of Fame credentials.