December 10, 1984:
Gary Carter for Hubie Brooks, Mike Fitzgerald, Herm Winningham, and Floyd Youmans
 
In their book It Ain't Over 'Til It's Over, the guys from Baseball Prospectus analyzed the twenty most lopsided deadline deals in major league history using Wins Above Replacement Player (WARP) as their yard stick. The deal that sent pitcher Len Barker from the Indians to the Braves for outfielder Brett Butler, third baseman Brook Jacoby and pitcher Rick Behenna on August 28, 1983 was deemed the most lopsided. Barker contributed only 3.1 WARP over the remainder of his career (2.4 with the Braves), while Butler (94.9), Jacoby (42.5) and Behenna (-0.3) contributed 137.1 over the rest of their respective careers, for a gain of 134.0.

The Carter deal didn't make the top 20, so we're going to use their system (and WARP scores) to determine which team came out ahead in the deal.

 
WARP Scores
YEAR CARTER BROOKS FITZGERALD WINNINGHAM YOUMANS EXPOS DIFF
1985 10.4 4.1 1.4 2.5 1.8 9.8 0.6
1986 8.4 5.6 2.7 0.6 5.1 14.0 5.6
1987 5.0 1.9 0.8 1.0 2.7 6.4 1.4
1988 3.4 5.6 1.3 0.5 1.5 8.9 5.5
1989 0.7 4.7 2.2     6.9 6.2
1990     4.6     4.6 4.6
1991     0.5     0.5 0.5
TOTALS 27.9 21.9 13.5 4.6 11.1 51.1 23.2
AVG/YEAR 5.6 4.4 1.9 1.2 2.8 10.3 4.7
 
Carter played for the Mets for five years and contributed 27.9 WARP, 37% of which came in his first season and two-thirds of which came in his first two seasons. It was mostly downhill from there. Brooks was almost as valuable to the Expos, producing 21.9 WARP in his five seasons in Montreal. Adding the other three gives Montreal a clear edge as they produced 10.3 wins above replacement player per season while Carter was worth 5.6 on average for the Mets. The Mets won a championship with Carter behind the plate, while the best the Expos could manage from 1985 to 1989 was a third place finish, so I'm sure the Mets were more than satisfied with the deal, despite Carter's rapid decline.

None of the above did much after leaving the Mets and Expos. Carter produced 7.7 WARP in his last three seasons; Brooks produced 6.1 over his last five; Fitzgerald produced 0.9 in his one season with California; Winningham produced 4.5 over his last five years; Youmans produced -0.5 WARP in his lone season in Philadelphia.

In a nutshell, the Mets mortgaged their future for a chance to win a title and it paid off. From 1985, when Carter joined the team, to 1990, the Mets averaged 96 wins a season and never finished lower than second. Only part of that can be attributed to Carter's arrival, but it shows that the team hardly missed the players they shipped to Montreal to get Carter.

The Expos traded an aging divisive force for younger players with potential and they came out ahead on the whole, but really received only one worthwhile player in return. Neither team parlayed any of their players into much talent in future deals either. Carter was released by the Mets, Brooks and Fitzgerald left Montreal via free agency, Winningham was traded to the Reds for 141 at-bats worth of Tracy Jones, who, in turn, brought the Expos two bad years of Mike Aldrete, and Youmans brought the Expos two crappy years of Kevin Gross.