I really like all eras of the band, but I am more of a fan of their more "fairly-tale"-like pastoral sound which was so strong in their 70's albums.
While I consider an album like Invisible Touch amazing due to its clever compositions, for example, I miss that atmosphere but then I listen to post-70's Genesis when I'm in the mood for something different than the 70's era. They were amazing throughout the many styles they tried.
The question is when do you think that last shades of this pastoral side, more fantasy world, were still present in their music, before they got a more "urban" sound? (Not sure if that's the best way to express what I have in mind, but I guess you got my point.
I think Abacab still features this sound (Me and Sarah Jane, Dodo, Like it or not, even a small bit in No Reply at All where Phil delivers some emotional vocals), and Three Sides Live maintain that atmosphere especially in that fourth live side.
However, begining from the s/t album, the mood seems to change drastically and that fantasy feeling disappears, even in the proggier moments such as Home by the Sea, Domino, etc.