My suggestion is, of course, Meteor.
The wrinkle would be in the application. I would like to see it be reminiscent of the opening video of the game, where a bunch of BLM are all casting what looks like meteor at the same time.
The wrinkle would be that if a single BLM casts the spell it would do moderate damage for extensive mana cost. However if multiple BLMs cast it simultaneously the damage would go up exponentially. For instance, single BLM damage is 500 points of damage unresisted. Two BLM casting would be 1000 points of damage each for total of 2000 points of damage. Three BLM casting would be 1500 points of damage each for total of 4500 points of damage. Six BLM casting would be 3000 points each for 18,000 points total.
Of course the numbers need to be tweeked and dialed in for balance, but you get the idea. May even need to limit it to BLMs in the same party or in the same alliance to keep it within reason.