I liked time magic best since I could perform the Stop Meteor trick which PWNS all.
Probably the most useful magic

I also like Time Magic, the powers to create Stop and Meteor. Powerful and cool Final Fantasy Magic moves they are.

the most powerful black magic spell is Ultima, Meteor, or Meltdown, which are my favorites. Meltdown, also known as Merton, is a magical ability, usually Fire-related, which is my favorite. It causes Fire damage and Sap. It deals unblockable, Defense-ignoring Fire/Wind dual-elemental damage to all targets on the screen, ally or opponent.

In Final Fantasy Xl, Meltdown is used by the Doll family of monsters, and is their version of a Bomb's Self-Destruct move. It deals Fire damage to all characters in a circular area centered on the Doll itself. In Final Fantasy Tactics Advance, Meltdown can sacrifices the user to do Fire damage in an area equal to their HP.

Anyways, I like all the magic overall, very usefull, but there are somes that are very extaordinary and magical.

I'm a fan of meteor, holy, and Firaga. Ultima is also pretty good.

Well, I'm going with Summonings for this one. I remember using all those summonings when I played FF3-FF6, especially playing with Bahamut, he kicked ass more then any other thing in FF3, he was awesome. Oh, and how can I forget all those other awesome summonings in those other games, those were kick ass too. So yeah, summonings all the way!
A close second for me would be White magic, they proved to heal and damage as well. Whatt's the use of having offence if all your warriors are at 1HP? White magic also can kick ass in terms of damage, like for example in casting spells like Ultima.
Overall though, Magic is a strong and wonderful thing, many people will agree. All types of magic kick ass.

I never really played too much FF. But I'm sure the magic on FF is the same kind on other games. So I chose Summoning, because I used to play Morrowind: Elder Scrolls 3. And summoning Daedra and other monsters was always a good strategy move because it makes most battles a 2 on 1 situation.

Blue magic is my favorite because of FF7. I know it didn't actually call it blue magic, but the enemy skill materia gives you enemies' moves which is the definition of blue magic. The E. Skill materia pwned everything, even Sephiroth. Of course Knights of the Round does better but still...