Definately Horizons for me.
Journey into Imagination never had a chance to spark mine, Spaceship Earth didn't really reach its full potential until the Irons version, the original Universe of Energy was even less interesting than the current version, The Land was missing a strong, memorable focus, and I never got to ride World Of Motion. The Living Seas had some really nice theming and memorable animals and exhibits, so it probably comes in second place by default.
Just curious, what was it about the Irons version of SSE that you felt was missing with the predecessor?