Giancarlo Stanton, 2017
Technically, the Cardinals traded for Giancarlo Stanton during the 2017 offseason. He just refused to waive his no-trade clause for them. And that decision had so many ripple effects for both clubs.
Stanton did not want to go to St. Louis or San Francisco and was later dealt to the New York Yankees instead, who took on about $270 million of the remaining $295 million on his deal. While Stanton is a fun player, his contract would have been a huge burden for the Cardinals.
From 2018-2023, Stanton has slashed .255/.339/.501 with 115 HR and 310 RBI. When he's healthy, he's still a mammoth bat in the lineup, but his health is almost never there. He's averaged about 100 games played each season for the Yankees and is pretty much a DH only for them. Although the Cardinals have the DH now, 2018-2021 would have been rough.
The Cardinals would then turn around and send the same package they offered for Stanton to the Marlins for Marcell Ozuna, who was one of the best hitters in baseball the year prior. The deal included future aces, Sandy Alcantara and Zac Gallen. At least they didn't trade them for a massive contract like Stanton, I guess.