I can list a lot of video games that I consider excellent, and I would have an extremely hard time choosing a single favorite. But I can say that I have never played a game that I enjoyed more than Crystalis.
I discovered Crystalis when trying to buy Ultima: Exodus. Ultima was only $15 and I had heard good things about the series. My copy turned out to be defective, so I exchanged it. After my third defective copy, I decided to try something else. Crystalis looked interesting, and was also $15. The box art and the price convinced me to try it, and I soon learned that the cheapest game in the store can also be the best.
I've played other games that occupied my thoughts and influenced my drawings often during and after the time that I played them. But I had a stronger emotional attachment to the characters and the story of Crystalis than with any other game I can remember. If you knew my fanaticism toward my favorite video games, you'd know that's significant.
SNK did not produce many other games for the NES. When I acquired a Super Nintendo, I researched carefully, trying to find something that compared to Crystalis. The closest I found was Zelda: a Link to the Past, an amazing game in itself, but I never did find another Crystalis.
Now, why am I saying so much about my experience with the game and nothing about the game itself? Well, why not? I'm sure you can find descriptions and reviews elsewhere. This is simply my acknowledgement of something I value greatly.