June 15, 1977: Tom Seaver for Pat Zachry, Doug Flynn, Steve Henderson and Dan Norman
 
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.

Since the Seaver deal didn't make the top 20 (believe it or not), we're going to use their system (and WARP scores) to determine which team came out ahead in the deal.

 
WARP Scores
YEAR SEAVER HENDERSON ZACHRY FLYNN NORMAN METS DIFF
1977 6.7 4.3 1.5 -1.8 .2 4.2 2.5
1978 7.1 6.6 2.7 0.6 .6 10.5 3.4
1979 5.1 5.2 0.8 2.1 .7 9.0 3.9
1980 3.3 6.3 3.0 3.3 -0.3 12.3 9.0
1981 6.8   1.4 3.0   4.4 2.4
1982 -0.1   1.6     1.6 1.5
TOTALS 28.9 22.4 11 7.2 1.2 41.8 12.9
AVG/YEAR 4.8 5.6 1.8 1.4 .3 9.1 4.3
 
Seaver pitched for the Reds from 1977 to 1982 and contributed 28.9 WARP over those six seasons. Zachry spent the same amount of time with the Mets and contributed only 11 WARP. Flynn was with the Mets for five seasons and contributed 7.2 WARP, and Henderson and Norman were with the Mets for four years and contributed 22.4 and 1.2 WARP, respectively. The Mets players as a group contributed 41.8 WARP vs. 28.9 for Seaver, a difference of 12.9.

You could argue that Henderson alone was worth as much to the Mets as Seaver was to the Reds. In their head-to-head matchup, Cincinnati gained a clear advantage in 1977 and '78, but Henderson contributed more WARP in '79 and '80. Over those four seasons Seaver averaged 5.55 WARP vs. Henderson's 5.6. When you add Kingman into the mix (Henderson was traded to the Cubs for Kingman in 1981), that adds an additional 5.4 WARP to the Mets' total in 1981 and 1982. Henderson and Kingman gave the Mets 27.8 WARP from 1977 to 1982, falling just below Seaver's 28.9.

That doesn't mean the Mets got the better end of the deal, however; it simply means they fared much better than they're given credit for. A closer look at the deal shows that Cincinnati gave up only one player that was making a contribution to their team. Zachry was part of the rotation, Flynn was barely playing, and Henderson and Norman were in the minors. Flynn wouldn't have cracked an infield of Joe Morgan, Pete Rose and Dave Concepcion and neither Henderson nor Norman were going to start in the outfield over George Foster, Ken Griffey and Cesar Geronimo. Henderson was a better hitter than Geronimo, but Cesar was a four-time Gold Glove winner.

So, in essence, the Reds traded a regular pitcher and three spare parts for a Hall of Famer. Seaver was consistently more than five WARP per season better than Zachry and he made the Reds a better team, without a doubt. The Mets, on the other hand, lost a great pitcher and had to settle for a lesser one in Zachry, but they improved in left field and at second base, and if you follow the chain, they not only traded Henderson for Kingman (an even swap in terms of WARP), but they included Flynn in a deal to acquire Jim Kern, who then went in a deal to acquire George Foster in 1982.