DUNGEONS & DRAGONS
Likely to be on a lot of peoples' A to Z, but I'd be telling a lie if i put anything else here. Sure things like dinosaurs and dark future might just as easily go here, but in the general A to Z of my diverse hobby, this simple box is likely the most important "D" there is and it has two of them to boot.
For me D&D is the big black monolith to my hobby. I had an imagination and an enjoyment for fantasy and adventure before that, but finding that box and reading those rules was what led to me throwing the dice into the air in slow motion and beating my geeky chest as it opened up the broad horizon of a lifetime of fun and enjoyment.
In a true Kubrickian moment, fantasy and adventure wasn't just something I did during play time with my friends, it was now a hobby, with focus, purpose, growth and expansion. It helped me express myself. It helped me learn (yes, it really did). Even my mother recognized how it changed me. I had always liked learning before D&D but after D&D I came home from the library with arms loaded with books on any number of subjects.