Deadline

My dad bought a copy of this when I was about eight years old. I spent the next three years trying to complete it, and the following twenty annoyed at the fact I never did.
Eventually, I got to download it again and finally completed it last year.
I've only ever played one other IF game (so far) so I don't have much to compare it to, but I've never played anything else that kept me hooked for twenty years.

For me, the characters are believable, the plot makes sense and everything you do has some kind of purpose (rather than being some random action). It's difficult though, because there are some things that have to be done at certain times.