To determine the best single seasons by Mets starting pitchers, I looked mainly at innings pitched, ERA, ERA+ (park and league adjusted ERA), and FIP (fielding independent pitching). Complete games, shutouts, K/BB, etc. were also considered. Playoffs were not. The list is dominated by Tom Seaver, as expected, although Dwight Gooden also makes multiple appearances and some recent performances made the cut.
10. David Cone, 1988
231.1 IP, 2.22 ERA, 146 ERA+, 2.58 FIP
I really wanted to include Bret Saberhagen's awesome 1994 strike-shortened season (he posted a ridiculous 11.00 K/BB), but Cone's season wins out. Cone was a strikeout machine early in his career, and is still the Mets all-time leader in K/9.
9. Al Leiter, 1998
193 IP, 2.47 ERA, 170 ERA+, 3.15 FIP
Leiter's first season with the Mets was his best. He might be higher on the list if he pitched more innings.
8. Johan Santana, 2008
234.1 IP, 2.53 ERA, 166 ERA+, 3.51 FIP
As I've covered elsewhere, Santana was a bit lucky in 2008. However, he led the league in IP and ERA and pitched one of the best games in Mets history on Sept. 27, 2008.
7. Jon Matlack, 1974
265.1 IP, 2.41 ERA, 149 ERA+, 2.42 FIP
Matlack is one of the most underappreciated Met pitchers ever. The big lefty easily could have had multiple seasons on this list. He won the Rookie of the Year Award in 1972 and was a big part of the Mets run to the 1973 World Series.
6. Tom Seaver, 1969
273.1 IP, 2.21 ERA, 165 ERA+, 3.11 FIP
Entries 4 through 6 are pretty interchangeable here, but I put Seaver behind Gooden and Jerry Koosman because of his FIP. That 1969 Mets pitching staff was ridiculously good.
5. Dwight Gooden, 1984
218 IP, 2.60 ERA, 137 ERA+, 1.69 FIP
Darryl Strawberry and Gooden won back-to-back Rookie of the Year Awards in 1983 and 1984. Gooden should have won the Cy Young Award in 1984 as well, but he would more than make up for it in 1985. His ERA would have been better in 1984, if not for a somewhat unlucky .313 BABIP.
4. Jerry Koosman, 1969
241 IP, 2.28 ERA, 160 ERA+, 2.67 FIP
In Mets history, Koosman is 2nd in IP, 2nd in complete games, and 2nd in shutouts. One guess who is 1st in those categories.
3. Tom Seaver, 1973
290 IP, 2.08 ERA, 175 ERA+, 2.57 FIP
Bill James once said there is an argument that Seaver should be regarded as the best pitcher of all-time. That may be a stretch, but he was incredible. He still holds the distinction of receiving the highest % of Hall of Fame votes, being named on 98.84% of ballots.
2. Tom Seaver, 1971
286.1 IP, 1.76 ERA, 193 ERA+, 1.93 FIP
See entry #3.
1. Dwight Gooden, 1985
276.2 IP, 1.53 ERA, 228 ERA+, 2.13 FIP
I think few could argue that this isn't the best single season in the history of Mets starting pitchers. Doc's numbers in 1984 and 1985 are just mind-blowing, and I wish the Mets could have limited his IP in those early years. That was a different era though. Hopefully Doc can keep himself on the straight and narrow, and make some Citi Field/SNY appearances as Strawberry has.
Honorable Mention: Bret Saberhagen - 1994; Pedro Martinez - 2005; Seaver - 1968, 1970, 1975, 1976; Koosman - 1968, 1976; Matlack - 1972; Craig Swan - 1978; Frank Viola - 1990; Sid Fernandez - 1992; Rick Reed - 1997; Mike Hampton - 2000