Digimon World combines Virtual Pet raising with a sprawling island adventure.
What made it unforgettable, fun and challenging?
Mystique.
The raising system is obtuse, and lacks positive reinforcement loops. Prior to data mining, the concensus on it's systems was widely disputed, and you had to work hard for every scrap of insight you had.
Despite, or perhaps because of it's difficulty, the process of raising your monster captures the imagination, and the presentation builds tension every time you reach point of digivolution.
The story, while light, is presented with grandiosity though rumours and old half-forgotten tales. The grungy ecclectic music perfectly matches the varied biomes of the world, which itself makes incredible use of pre-rendered backgrounds through the shot compisition for every screen.
Low angles reveal the grand vistas and create questions about the workings and creation of the world, with environments fusing the natural, the ancient and the technological. High angle over-head views limit visibility, and create tension between the fun of exploration, and the danger of the unseen. And the consistency of the environment, even with the implied gaps between screens, gives the world a real sense of tangibility.
The City graduadually grows, like a little hodge-podge diorama of (in some ways literal) kitchen sink design. While some Quest Designs are simple battles, many are more lateral, and reward outside the box thinking, giving the game a hand-made quality often lacking in modern big-budget game design.
Digimon World is far from a perfect game, sorely lacking in the QOL features we'd expect from modern design. Me and my brothers never even managed to beat it back in the day. But it's weird, unique, mysterious, dripping with atmosphere, and, warts and all, worth remembering.