I always buy organic, which also means non genectically modified soy. I can recall having a weird taste from one I bought some time ago, and all I remembered was it wasn't organic. I stick with House Foods Brand from an Asian grocery, but I know it's at the regular store, just costs more.
I do find the aftertaste in plain soymilk, though, but chocolate is delicious!
Don't let the perfect be the enemy of the good