Mike Piazza to the Marlins
This move is on most folks' list given Mike Piazza's stature as one of the better catchers to ever play the game, but also because of the sheer scope of the trade itself. In 1998, Piazza was at the top of his game and it was clear that negotiations on a potential extension were not going well. Instead of just letting him walk, they traded him to the Marlins for Gary Sheffield, Bobby Bonilla, Jim Eisenreich, Manuel Barrios, and Charles Johnson. The Marlins would then flip Piazza to the Mets for Geoff Goetz, Ed Yarnall, and Preston Wilson.
The Mets clearly ended up the winners here as they got a Hall of Famer for the better part of a decade. The Marlins and Dodgers? Yeah, not so much, although Sheffield did have some nice seasons in LA. It's still funny that Bobby Bonilla is still getting paid by the Mets and will until 2035.
Gary Sheffield to the Braves
Speaking of Gary Sheffield, he gets the nod next. After three straight excellent seasons with the Dodgers, Sheffield wanted out of Los Angeles if they weren't going to commit to him. He got his wish when he was traded to the Atlanta Braves for Brian Jordan, Odalis Perez, and Andrew Brown. This one isn't completely on the Dodgers since Gary was pretty clearly trying to force his way out, but it still feels bad.
Sheffield would go on to play two very strong seasons in Atlanta and then continue to rake with the Yankees before his borderline Hall of Fame career tailed off with the Tigers and Mets. LA did get some decent work from Odalis Perez at the beginning, but Brown didn't go anywhere with the Dodgers and Brian Jordan was only mediocre in 1.5 seasons of work before he moved on to the Rangers and then finally back to the Braves.
Pedro Martinez to the Expos
Yep, if you have followed the Dodgers for a while, you knew this one was coming. While we don't have hard and fast rankings here, this one is likely on the top of everyone's lists of worst trades in Dodgers history. Pitchers are always tricky because they are very clearly the most volatile of any type of player and the most likely to flame out completely. But back in 1993, the Dodgers traded with the Expos to get Delino DeShields, who was a perfectly reasonable player to want on your roster. Who did they send to get him? Pedro Martinez. Yes, that Pedro Martinez.
DeShields wasn't terrible, but Pedro's career speaks for itself. He has a very strong claim to being the best pitcher of his era after racking up three Cy Young Awards, multiple ERA titles, eight All-Star Game appearances, and of course being enshrined in Cooperstown in his first year of eligibility back in 2015 with 91.1% of the vote. He's one of the best to ever play the game and was a Dodger once upon a time. Oh, what could've been.