Oh god. Mine would have to be the famous +RES -ATK or +DEF -ATK. And this is on units that don't have really good DEF or RES.
Perfect Examples of +DEF -ATK: Legault, Julia, Azura, Ishtar, Linde, Deirdre, WoT Olwen, Brave Lyn, and Legendary Lucina. Lot of Women for some reason.