Not the earliest you still play regularly. Just the first.
Mine would have to be Colossal Cave Adventure, followed by Scott Adams' Adventureland. I played the former, but saw the latter in!color! on the brand new Apple II purchased by a couple of friends. As we were all writers, a merry time was had kicking the shit out of the mechanics and parser. 
I recall they got it as a freebie (one of several) with the computer purchase. This was back in the late seventies. Had we known then what we did now, we would have invested in the hardware. But then, there were plenty of opportunities for that, that turned into dead ends, too.
Over to you. First computer game you ever played.
