KH1 is my favorite in the series. By far? No, but still my fave. Why? Immaculate vibes (i.e. too long to explain). And it's nostalgia either, because I actually only played it myself as an adult, KH2 and Chain of Memories were my firsts. There's something so incredibly serene, magical, and mysterious about KH1. And it has by far the best end world: The End of The World. The mysterious silent voice that talks to you every now and then, and to Sora too (Sora acknowledging it in The Final Sanctuary made me feel something). It's something I have not felt at all in the other titles. And the nice, simple story and the charm of it all. Heck the characters were all at their best in KH1, it was downhill from there for everyone (Kairi lost her entire personality, Sora got dumb and lost his smartass boy attitude, Riku got depression then lost his personality too, etc). And Billy Zane as Ansem is pretty much orgasmic to hear. Guh. I love it so much.
And it's not nostalgia either*