FF4's boss theme
FF6, Metamorphosis
FF7, Birth Of A God (Bizarro Sephiroth)
FF8, The Extreme (Ultimecia's last form)
FF9, Main battle, Trance Kuja and the final battle
FFX's normal battle theme, though I realise it'd be hard to make it sound right... I can't play it through in my head like the others.
FFX-2, Rikku's Theme (If you can't remember her, get her to use a Sing ability)

And a metal version of Eyes On Me, preferably NOT sung by a female J-Pop idol. The song's in English, please, please pick a singer who normally sings in that language. Hearing it done by some scandinavian deathmetaller would be hilarious actually.

By the way people, Matoya's Cave has been done already. It was a very neat blues piece. I really liked it.

A Black Mages arrangement of FF3's final battle theme is on the DS soundtrack as a bonus.