18? I guess I'd try and pick two tracks each from 4-10 & 12 then fill in the gaps.

1. IV The Dreadful Fight (Four Fiends Battle theme)
2. IV Tower of Babil
3. V Battle with Gilgamesh
4. V The Dragon Spreads It's Wings
5. VI Cyan's Theme
6. VI Forever Rachel
7. VII On The Day, 5 Years Ago
8. VII Still More Fighting
9. VIII Truth
10. VIII Julia
11. IX Freya's Theme
12. IX Hunter's Chance
13. X To Zanarkand
14. X Illusion
15. XII The Forgotten Capital
16. XII The Sochen Cave Palace
17. VII Cid's Theme
18. Final Fantasy Main Theme

Dear God, 18 tracks is terribly limiting! I don't even think the 18 I've picked entirely do the music justice. I could probably do with 18 tracks per Final Fantasy.