Pretty much all of them. Start with the 1.5 2.5 collection. Here's the play order:
- Kingdom Hearts Final Mix
- Re:Chain of Memories
- 2 Final Mix
- 358/2 Days (Cutscene Movie)
- Birth By Sleep Final Mix
- Re:Coded (Cutscene Movie)
If you like them, then get 2.8 and KH3.
as someone who isn't very familiar with Kingdom Hearts, which games are absolutely necessary to play and to understand the story?