So after beating 3rd, I can post my enjoyability ranking list.
1. Zero
2. Ao
3. Sky 3rd
4. Sky SC
5. Sky FC
6. CS2
7. CS1
It's really impressive how Falcom improves their work with each subsequent game within a series. At the same time, despite Zero having a weak final boss has an excellent style and presentation which is something that's not matched in any other Kiseki game.
The writing in 3rd is absolutely fantastic, and with the several standout Doors that eclipse almost any other Kiseki game, pushes it a bit above SC for me, which is tough because of SC's emotional highs, and it's tough to rank SC over FC's wonderful introduction structure.