Kings Quest
Not just cos I've got about 7 or 8 of them, but cos I've never finished one! Especially not the one where you're some wizard's slave trying to escape, wizard always zaps me, sigh. Just keeps me going back to them, when I could play my blasted Amiga that is. (Last playable <1995 when I was still a teen, since then I've completed the likes of Broken Sword and Discworld so not a complete moron!)