Fallout 1
It is highly replayable game, but it is not about whether or not I know its main story.
I've been playing both TTRPGs and CRPGs since 80s, but it was always FRPs (as we used to call them, Fantasy Role Play) - when Fallout launched in '97 it was revelation. Not only it used system that is very different than usual D&D mechanics, but its post-nuclear setting was completely different than what I was used to (and as big fan of Mad Max II, right up my alley).
Not knowing where to go, what to do, with danger being on every corner and 150 days counter ticking down, it got me to some fantastic situations on my first playthrough that I never experienced before or after. Oh, and that Mark Morgan's soundtrack that was perfectly capturing desolation of irradiated wasteland and was putting chills in my bones for the whole time.
It is nowhere near perfect game, but it is perfect creative storm of some great designers/artists who managed to make something truly unique on what was Interplay's B-tier project, and my favourite VG experience.
Essay Pro