Keep in mind that the "Normal" mode is actually the "I've never played a video game before" mode and the "Classic" mode is essentially a fatalistic grind towards eventual alien dominance of the world. I can't imagine what "Impossible" mode is like. All Chrysalids, maybe? I prefer Classic w/Ironman. Feels more like an alien apocalypse mini-series where the good guys don't really have a chance of winning without some great strings of luck/acts of god.
The way I remember, the original was insanely difficult, so "classic" sounds about right.