8. Forced To Kill Colossus
The Dark Phoenix Saga is arguably the single most iconic and popular X-Men story ever told (at least in print, as both film adaptations fail to live up to the comic).
The legendary collaboration between writer Chris Claremont and artist John Byrne created many classic X-Men stories and arcs that have not lost their impact over four decades, and have been adapted into blockbuster films (some more successfully than others).
The impact of the Dark Phoenix Saga on Wolverine is perhaps more than the impact on any of the other characters combined. Although this particular event occurred in an alternate timeline, one of the most devastating moments for the clawed crusader in the Dark Phoenix Saga is when he was forced to kill his friend and fellow X-Man, Colossus, because of the Phoenix.
Again, this wasn't actually part of the Dark Phoenix Saga; just an alternate-universe spin-off story from 1981's What If #27. Even so, its psychological impact on Wolverine was likely immense, and as it wasn't overshadowed by Jean Grey's subsequent death in this timeline, this is surely one of Wolverine's most surprisingly emotional moments.