Everyone says Fruits Basket is sad... I don't...
A. Everyone has a sad past, but Tohru is definitely gonna bring the family a brighter future.
B. The anime ended with everyone still living together.
Yes, I agree with Akai Yuki, X and Tokyo Babylon are few of the saddest anime/manga. People suffer in Fruits Basket, but they are still alive, they have a chance to turn it around.
Sorata lost his chance to develop his relationship with Arashi just when it was about to begin; Kakyou is never gonna wake up because Hokuto's not coming back; Kamui will always be a beaten- up dog because the girl he likes turns into french fries (chop, chop chop); worst of all, Subaru is going to live all alone since Hokuto is dead and now Seishirou, too (presumeably in hell for him).
People in X/Tokyo Babylon died, and they are not coming back, yet their existence is what the surviving characters need to fill in the void in their souls. Unlike Fruits Basket w/ Tohru around, nothing in X/Tokyo Babylon can help repair the damages being done.
Well... unless the author of Fruits Basket decides to hit the characters where it hurts, cut off their sole spiritual comfort- kill off Tohru at the end... NO, NO, NO, She would not dare...
Oh, btw, There's another sad CLAMP- RG Veda. Very much like X, Ryu-chan is the first, kinda unexpected... not only that almost 99% of the characters were killed (brutally, I might add), not a lot of god races are left. Either they end up like Yasha, Souma, and Asura, losing everyone but the lords, or like Kendappa, Naga, and Karura, there are still people, but they have no leaders. Six stars overthrew Taishakuten, but the whole world is a mess even until the end...