For me it was 358/2 Days. My very first Kingdom Hearts game, I've never even touched 1 or 2... The way how everything fell apart for Roxas was just REALLY depressing!
358/2 days. First Xion gets a new friend (Roxas), Then she becomes really concerned about her existence? Roxas has no idea about his existence either, but he stays calm. Everything just fell apart for Roxas. The part that was very stupid is when Riku defeated Roxas. If Roxas is Soras nobody wouldnt he be able to defeat Xemnas?
If i made the game i would change the storyline a little.