Even Better Sequel: Weather Systems to We're Here Because We're Here. Reviews describe it as taking everything that made WHBWH good and going even further.
The concept of The Optimist. Whereas A Fine Day to Exit was the loose story of a man being pushed to brink of suicidal depression, The Optimist is about his newfound will to live and his journey home.
Signature Song: "A Dying Wish" and "Fragile Dreams" from their metal period, "Untouchable, Part 1" and "Thin Air" from the Kscope years.
Even before that, there was "Violence", the 10-minute instrumental piece that concludes A Natural Disaster. Coming after the forlorn "Electricity", it's also a massive Tear Jerker in context.
Tear Jerker: So many of their songs are this trope that it's hard to choose a specific one as an example.
The music video for "Dreaming Light", the pinnacle of the mood and genre shift.
All three parts of "The Lost Song", but mainly Part 1 and Part 2.
Both parts of "Untouchable".
They Changed It, Now It Sucks!: Their shift from death/doom/gothic metal to atmospheric rock had this effect, possibly even more so than the various shifts in genre Paradise Lost made.