Firestorm is the best 19th level area damage spell. But let's compare it to the two level 19 zones wizards can produce, Evard's Black Tentacles and Cloudkill.
Size/range: Firestorm is a burst 5 within 10, Evard's is a burst 4 within 10, and Cloudkill is a burst 5 within 20. Cloudkill wins here for its largest range, with Firestorm taking a close second. Evard's smaller size at the closer range puts it in third.
Targets: Firestorm wins this category hands down, as the only one of the three that won't hit allies. Cloudkill comes in a distant second, as the wizard will have the opportunity to move it off of any allies before they begin their turn in it.
Initial damage: 5d10 for Firestorm, 2d10 for Evard's, 1d10 for Cloudkill. Another solid win for Firestorm. Firestorm also does half damage on a miss, and is the only one which does so.
Additional effects on hit: Evard's will immobilize those it hits. This category goes to it, as the only status effect available.
Ongoing damage: Firestorm and Cloudkill cause 1d10 damage to each target in the zone each round. Evard's causes 1d10 to immobilized targets each round, and has a 50% chance to cause 2d10 to non-immobilized targets. Call it a tie.
Zone: Evard's wins here, as it is the only zone that has any effect other than damage. It turns the zone into difficult terrain.
Escapability: Most creatures can get out of Firestorm easily. A concerted effort on the part of the enemies can move the conflict point to a part of the battlefield not in the zone. Evard's is much harder, as a creature with speed 6 in the center cannot get out with a single walk action. They can run, granting combat advantage, or use their standard action for more movement. And that assumes that they aren't subject to the immobilize effect on their turn. It's easy to get out of Cloudkill, but hard to stay out, as the wizard can move it up to three spaces on his turn. Note that a creature at the center with 6 speed can't get away from it with even with a run. They must double move to be certain to be clear of the effect. Even if they do get clear, they have to keep moving if they don't want the zone to catch up to them. The immobilization effect makes this go to Evard's, but Cloudkill is a close second.
So which is best? Well, Firestorm is best for blasting the snot out of the enemy. Cloudkill is the best at keeping the enemies from entrenching into a defensive position. Evard's is the best at creating a region of sticky, tentacular horribleness. It's all a matter of taste, really.