I don't consider it the best ever made, personally, but it stuck with me most because of the way it handles its tone and exploration. The tone is carefully constructed to truly feel dream-like when you're in the right mindset/environment to fully take it in, with particular design choices distorting the player's sense of time. That makes it a unique experience at least, as there's hardly anything else that feels similar to me. (Maybe not nothing, but very few other things.)

My favorite aspect is the exploration. There are so many weird and cool things to discover, some of which are really hard to find and feel special because when you stumble upon something cool, you may not have any idea how to find your way back there, so especially for a first no-walkthrough run, it creates a strong sense of unpredictability. And some areas or events are so obscure that it creates a sense that there's always more to discover.

It's also uncompromising in the way it handles its gameplay. It's minimal, and if it compromised to give the player a challenge, dialogue, or specific objectives, it'd change the tone or take away from the joy of just exploring for its own sake. Of course, it's not perfect. It's clearly the product of someone just making a game for fun and not expecting it to be popular, but that gives it a certain charm. It's almost like a zen experience, a good game for not having to think much and just taking it all in.

And finally, it's pretty influential. Many or even most of the RPG Maker horror games are inspired by it directly in some way, and even some games in completely different genres. While there might be a lot of games in a similar vein nowadays, back when it came out in 2004 that wasn't the case, pretty much the only thing at all similar was LSD Dream Emulator and maybe parts of Earthbound