Well, what constitutes a perfect game is completely subjective, so I'd have to say the perfect game is the first game you played and absolutely adored. No matter what comes after it, nothing's ever going to live up to the memory of that amazing game.
For me, it's Kingdom Hearts. I play through it at least once a year, and it still gives me chills. I never quite recaptured that sense of amazement and wonder I had when I played it for the very first time, but to me, it's still very much a perfect game.