I really enjoy FF games for the main reason of having a wide variety of choices to disable an enemy. So, the ability to choose between Flare, Meteor, bahamut, and holy really makes the game more entertaining.
I don't really like meteor since the time it takes to cast it lets the player go for a smoke
. However it simply mauls the opponents with (i think) a guaranteed damage of 9999, so i must admire its power.
Flare was like the invisible boy who sits in the corner quietly..hehe. Mainly because i never used it more than 2 times. Once to see its potential, and second to conclude that there are better options.
BTW, the word mega implies its following word to be multiplied by one million as dictionary.com
http://dictionary.reference.com/browse/mega states. Therefore it is weird that bahamut's mega flare only inflicts 5000 to 9999 where a simple flare may inflict from 2000 to 9999 damage. Technically, mega flare should inflict from 2 000 000 000 to 9 999 000 000. But, i mean c'mon....thats way too...real. The main point i am trying to make is that by adding certain words before spells causes the player's mind to assume that it is more powerful than the spell without the word. That is why mega flare > flare IMO.
Anyway, i digress so back to the topic.
Holy, which was pointed out by Dawezy, is truly one of the badass spells of FF4. Its potential to cause well over 4000+ is decent, however it really wipes the floor with undead (dark) monsters. The addition of this spell to a weapon of cecil, the Lightbringer, is also really nice. The only con of this spell is that some enemies absorb it such as Zemus' face and Zemus' heart or something of the sort.
The ideal spell to me is definitely the Bahamut summon. The main reason being because 'twas the most used spell in my recent run through the GBA version. Also, towards the end of the game where Rydia is around level 60, it mostly always inflicts 7000+ of damage. Seriously, who doesn't love to see a massive dragon drop from the sky (Well...appear from the right in FFIV) and unleash a Mega Flare.
Long live Bahamuts in FFs