One of the rarest games I've played is King's Quest 4 oddly enough. Or rather a particular version of King's Quest 4. What made this particular KQ4 so rare is that it was built using the older Sierra AGI engine as opposed to the later SCI engine.
Sierra originally had intended to release the AGI version for those with lesser hardware. The SCI version required at least an 8 MHz CPU, used a 320x200x4bit (16 colour) graphics display, had full sound card support and required 512KB of RAM (these were steep requirements back in the day), while the AGI version used a 160x200x4bit graphics display, had at best 3 voice Tandy/PC jr. sound support and only required 256KB of RAM.
Honestly I preferred the AGI version, it felt more like a "real" Sierra adventure game. Sometimes better graphics can detract from the quality of a game.
Sierra originally had intended to release the AGI version for those with lesser hardware. The SCI version required at least an 8 MHz CPU, used a 320x200x4bit (16 colour) graphics display, had full sound card support and required 512KB of RAM (these were steep requirements back in the day), while the AGI version used a 160x200x4bit graphics display, had at best 3 voice Tandy/PC jr. sound support and only required 256KB of RAM.
Honestly I preferred the AGI version, it felt more like a "real" Sierra adventure game. Sometimes better graphics can detract from the quality of a game.