2nd Ed AD&D
or
Magic the Gathering
(yes, neither of those are digital. Can't think of a videogame that I'd take as "only game ever".

)
...and, honestly, neither of those are good choices, either. I bounce around between games too much. There's no way I could just play one game. I'd get sick of it inside 6 months, and then not play for a year, then play it again, then probably get sick of it forever.