I think that it'd have to be something with a lot of replayability, which doesn't lose value to me after one playthrough.
Also, it can't be a genre where the game was limited by technology. I mean, I remember Wolfenstein 3D being amazing when it came out relative to other games of the time -- walking around in a 3D world was so mind-blowing -- but the novelty of that technology has long-since worn off, and there are many more-impressive 3D games today.
I guess roguelikes are probably about the top of the heap there, and my favorite is probably Cataclysm: Dark Days Ahead. I still regularly play that, which seems to me to be a good test of whether it's still at the top of my list.
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