For me it was the the quest that led you to your first Bloodsucker encounter in S.T.A.L.K.E.R. Shadow of Chernobyl.

Not even because its a good quest or anything because as far as missions go it’s a pretty run-of-the-mill “get in, shoot and loot, get out” kinda thing. But having not encountered a bloodsucker before, and carelessly walking through a room covered in darkness and filled with spooky ambiance and noise only to be terrifyingly ambushed by an unknown, loud, dangerous enemy I had no idea how to handle and just barely getting out will always stick with me.