Ultima: Escape from Mt. Drash
This obscure and legendarily rare entry in the popular Ultima RPG series was produced independent of creator Richard Garriott's knowledge. Only released for the VIC-20, it is a simple maze game with some monster battles, and very little connection to the other games in the series.
It's got Ultima in the title so I had to play it. There is a reason this title is so rare though - no one wanted to buy it in the first place. Unless you have to play every one, then avoid at all costs.