Quoted By:
>KH 1.5 Collection (KH1, Chain of Memories, 356/2 Days cutscenes)
>KH 2.5 Collection (KH2, Birth By Sleep, Re:Coded cutscenes)
>KH 2.8 Collection (Dream Drop Distance, 0.2 which is a prologue of KH3)
>KH3
Streamlined: KH1 > KH2 > Birth By Sleep > KH3
If you only have time to play one, play KH2 since it has the best gameplay of the series, especially on higher difficulties.