Flare or Holy

Meteor and Ultima aren't exactly what one would call economical spells; especially Ultima. A Black Mage may be able to learn those but Flare in and of itself is quite superior to them (with the exception of SOME meteors) as it ignores defense it isn't too costly and you would deal more damage by casting that twice than casting ultima once and still save mp.

Holy itself is rarely used and often not obtainable. in final fantasy 9 Holy was incredibly powerful and made Eiko an actual choice between Garnet and her. But overall if you have Flare; a non-elemental defense ignoring spell against a powerful but absorbable spell...Holy kind of falls short in my opinion.