If you mean any computer, then it was Adventure (with a capital 'A', and it's maze of twisty little passages all alike), on a Data General Eclipse mini-computer. Although, I might have played Hunt the Wumpus first. I can't remember clearly that far back.

Adventure is the game I remember best.
If you mean a personal computer, then it was Zork on my original Mac (128K, no hard drive).
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Nid yw'r hoff o lyfr yn fyr o gyfaill.