It all depends on the scenario.
Immolation: Good with weaker creatures such as photon or ash eater. Very dangerous when coupled with Fahrenheit or other quanta-dependent cards. Even better in the form of Cremation. Plus, unlike nova, it doesn't have limited uses per turn (at least not to my knowledge.).
Brimstone Eater: Like a burning pillar that attacks. Incredibly useful. It also only costs 1 quanta, adding to its usefulness.
Lava Golem/Destroyer: Not the best alone, but with an earth mark it works amazingly. Parallels and outside stat boosts make this card an even bigger force to be reckoned with.
Phoenix/Minor Phoenix: A glass cannon and a sentinel in the same card? Perfect! Has surprising endurance with its ability that allows it to revive when in Ash form. Minor Phoenix is even better considering the ability to be spammed more easily. Works amazing with fractal, proven by the False God "Eternal Phoenix".
Ruby Dragon: The definition of a glass cannon. Sure, it's 2 health is discouraging, and it costs 12 fire quanta, but that is more than made up for by its gargantuan 15 damage per turn. (7 hit kill when used alone) Couple this with cards that allow you to increase a creature's health, and you have a monster.
Fire Lance: Why did I pick this over fire bolt? It can be spammed easier. At 75 quanta, 6 fire bolts do 132 damage. Fire lance, however, increases that to 144 damage, thanks to it only costing 1 quanta instead of 3. Couple this with some creatures and Fahrenheit and you have a great strategy on you.
Fahrenheit: Probably my favorite fire card (and favorite card in general). I was so happy when I got this after completing the 500 score quest that I instantly upgraded it. At very high quanta levels, this does 20 damage, more than even Ruby Dragon!
Fire Storm: Like the difference between fire bolt and fire lance, I picked this over Rain. This is like Thunderstorm from the Air element, but it does 1 more damage for a higher quanta cost. This card has been used well in some strategies like Firecell and even in decent Mono Fires and Duals.
Deflagaration/Explosion: Destroys a permanent. Doesn't sound awesome? Shields like Dimen-Shield, Dusk Mantle, Procrastination, Unholy, Diamond Shield, Dissipation, and weapons like Discord, Arsenic, Pulverizer, Trident, and Eternity can all be destroyed by this one card.
Shard of Bravery (Also known as: SoB): This shard is used in a number of strategies, including the ever-so-infamous Voodoo Bravery. It is one of the more popular shards, and I concur to the global opinion. Speeding up the gameplay like precognition and golden hourglass was an amazing idea, especially considering now time isn't the only element able to do so.
Overall, my pick would be Fahrenheit because of its versatility and output, but all of these are great picks.