Thought this might be an interesting topic...
If you could go back in time and reverse 1 trade made by the Rangers over the lifetime of the franchise, what trade would you reverse and make never happen and the players involved still put up the same numbers for their original teams as they do on the teams traded to. So Sammy Sosa would still put up his Maris chase of a season and career, Travis Hafner still turn out the way he has, etc.
Myself, I think I might have to look hard at that Hafner trade. However, that would also mean no Nevin trade. In the end, I think I might have to go back the Sosa trade of Sosa, Wilson Alvarez, and Scott Fletcher to the White Sox for Fred Manrique and Harold Baines.
Baines and Manrique didn't provide much to the Rangers, Manrique gone at the end of the year and Baines traded to the A's the next season for 2 players to be named later. Meanwhile, Fletcher was a decent middle infielder, Alvarez went on to have a nice career as a starter that would have looked nice in a Ranger uniform, and we all know what Sosa went on to accomplish.