Example 14th level wizard with Int 22.
FireBall, 3rd Level Blast
Normal: 3rd level slot, 10d6 dmg, DC 19
Limited Magic: 3rd level slot, 5d6 dmg, DC 14
Constitutional: 4th level slot, 5d6 dmg, DC 19
Heightened to 6th level: 6th level slot, 11d6 dmg, DC 19 <-- Probably your best option.
Heightened and Constitutional: 7th level slot, 11d6 dmg, DC 23
Compare to Best 7th Level Blast Spell, but only one target
Finger of Death, 7th Level
Normal: 140 dmg, DC 23, save for 3d6+14
LImited, 130 dmg, DC 20, save for 3d6 +13 dmg
FireBall, 3rd Level Blast
Normal: 3rd level slot, 10d6 dmg, DC 19
Limited Magic: 3rd level slot, 5d6 dmg, DC 14
Constitutional: 4th level slot, 5d6 dmg, DC 19
Heightened to 6th level: 6th level slot, 11d6 dmg, DC 19 <-- Probably your best option.
Heightened and Constitutional: 7th level slot, 11d6 dmg, DC 23
Compare to Best 7th Level Blast Spell, but only one target
Finger of Death, 7th Level
Normal: 140 dmg, DC 23, save for 3d6+14
LImited, 130 dmg, DC 20, save for 3d6 +13 dmg