In the Buck thread, PghPinstripes linked to a CBS Sports series in which they're creating franchise all-time teams based on the best single season at each position. They're doing the Pirates on Wednesday, but I thought I'd beat them to the punch and do my own team. I'm trying to get this done tonight, so I readily admit I may miss a great season or two. I'm not including any pre-1900 seasons, and deadball era pitching presents its own problems. I used WAR (Fangraphs) as a rough guide, mostly with hitters, but didn't feel bound by it. The names link to bb-ref pages if you want to see the stats.
Catcher: Jason Kendall 1998
Kendall was headed toward a huge season in 1999 (3.9 WAR in 78 games) when the ankle injury intervened. That leaves 1998 (327/411/473) and 2000 (320/412/470) as his best seasons. He had a lot more baserunning value in 1998, obviously, but otherwise was largely the same player. His 2000 season shows conclusively that it was the 2001 thumb injury, not the ankle injury, that robbed him of his power. Manny Sanguillen (1971 -- 319/345/426) and Tony Pena (1984 -- mainly due to great defense) had 5.0 WAR peak seasons, but Kendall is the clear leader.
First base: Willie Stargell 1972
This is tough, because Stargell's best seasons came in left; he played first regularly only in '72 and from 1975 onward. But he did have a 164 OPS+ in '72 and drove in 112 runs, his third-best total. The best one-year WAR total for a Pirate 1B (5.6) appears to have been Elbie Fletcher in 1941. That was the middle of three years in which Fletcher led the NL in OBP. Gus Suhr in 1936 (312/410/467) came close at 5.5, and Kevin Young (298/387/522) and Jason Thompson (31 HR, 101 RBI) had big years in 1999 and 1982, respectively. Donn Clendenon and Dick Stuart were killed by defensive issues.
Second base: Johnny Ray 1984
The Pirates have gotten a lot of 4+ WAR seasons from second base, but no great seasons. Ray's best season was 1984, in which he hit .312 and led the league in doubles, and he still played good defense at that point. George Grantham was the best hitter the Pirates ever had at the position, but couldn't play defense. Claude Ritchey had a big season in 1903 and Rennie Stennett in 1975, both largely due to great defense. Bill Mazeroski's best season was 1958, but mainly he was a 3-WAR player.
Shortstop: Honus Wagner 1908
Wagner had a lot of incredible seasons, but in 1908 he hit 354/415/542 in a league that hit 239/299/306. His OPS+ was 205 and his WAR was a Pirates' record 11.8. Arky Vaughan's 1935 season (385/491/607) would be the best ever at short for nearly all other franchises, but Wagner's 1908 season may have been the most dominant ever relative to the rest of MLB by any hitter who wasn't named after a candy bar.
Third base: Tommy Leach 1902
The Pirates have had some pretty big seasons from third base. Leach in 1902 had a 134 OPS+ and, as best we can tell, was a terrific defensive player. He also led the NL with 22 triples and six (yeah, six) HRs. Freddy Sanchez led the league in average and doubles in 2006, but actually fell well short of Leach's 1902 season on a WAR basis. Bill Madlock's best Pirate season (341/413/495) was his first batting championship in strike-shortened 1981. That maybe should have been the top season, but he only played 82 games. Richie Hebner had a 153 OPS+ in 1972, but he was a platoon player. Aramis Ramirez' 34-HR, 112-RBI season in 2001 actually resulted in an OPS+ of only 122 in a high-offense environment. Finally, Hall-of-Famer Pie Traynor was more of a good, 3-4 WAR player than a great player.
Left Field: Barry Bonds 1990
This position produced a large percentage of the best offensive seasons in team history. Willie Stargell had two huge seasons (1971 -- 48 HR, 125 RBI, 295/398/638; 1973 -- 44 HR, 119 RBI, 299/392/646), Ralph Kiner had three (1947 -- 51 HR, 127 RBI, 313/417/639; 1949 -- 54 HR, 127 RBI, 310/432/658; 1951 -- 42 HR, 109 RBI, 309/452/627), and Brian Giles had a string of outstanding seasons. None of them came close, though, to Bonds' 1990 season (30 HR, 114 RBI, 52 SB, 301/406/565). A big part of the difference was the fact that, as good as he was defensively during much of his Giants' career, Bonds was much better before he had to lug around a 75-pound head.
Center Field: Andrew McCutchen 2013
This isn't close. The next best seasons by a Pirate center fielder other than McCutchen were probably Andy Van Slyke's 1988 (288/345/506) and 1992 (324/381/505) seasons.
Right Field: Roberto Clemente 1967
This is incredibly close. Clemente's best season (357/400/554 in a low-offense era), Dave Parker's 1977 (338/397/531, great defense before going downhill the following year), Paul Waner's 1927 (380/437/549) and Kiki Cuyler's 1925 (357/423/598 -- too bad Bob Nutting gave him away to get him off the payroll) were all nearly identical in value. I just chose Clemente because . . . Clemente.
I mostly gave up on WAR for pitchers, as I don't have much faith in it for pre-modern periods, so this is pretty arbitrary. Veale in 1965 (17-12, 2.84) set a Pirate record with 276 strikeouts (WAR likes his season a lot, too). Candelaria went 20-5 and led the majors in ERA (2.34) and ERA+ (169). Kremer went 19-8 and led the NL in ERA (2.47) and ERA+ (168). (Three years later he won 20 games with a 5.02 ERA.) Law (20-9, 3.08) won the Cy Young Award, led the majors in complete games, and broke the team's two longest losing streaks. Blanton led the majors in ERA (2.58) and WHIP, and the NL in ERA+ (159). Arguably the team's two best pitchers all-time, Babe Adams and Wilbur Cooper, had some very good seasons but none that stood out. Two other great seasons were Jack Chesbro's 28-6 season in 1902 and Howie Camnitz' 25-6, 1.62 season in 1909, but deadball era pitching stats are problematic. WAR loves Bob Friend's 1960 season (7.8 to Law's 4.9 that same year -- they had nearly identical ERAs but Friend's FIP was the better part of a run lower).
This is both easy and hard. Easy because there's basically nobody before the late 1950s, hard due to changes in usage patterns. Gossage was really easy, though, because his 1977 season (11-9-26, 1.62, 133 IP) was one of the best ever by a reliever. Face maybe had a better season in 1962, but in 1959 he went 18-1. Hanrahan had 40 saves and was ultra-reliable. The 1979 three-headed bullpen monster of Kent Tekulve, Enrique Romo and Grant Jackson deserves some sort of merit badge. The three combined for 250 games and 345.2 IP.