Fire

Fire pets hit for a share of your own attack on their timer and set the target burning: a short, sharp damage-over-time worth a large fraction of the pet's attack power in a single tick. Legendary fire pets also pierce part of the target's resistance when they apply the burn, which is what makes them the answer to something armored.

The shared tree's power investment goes straight into the strike. Fire also gates its burn on proc chance, and because legendary fire species already open at the engine's proc ceiling, anything the ceiling refuses is converted into extra strike damage instead of evaporating.

Builds that already win and want to win sooner. Fire adds nothing defensive, so it is the wrong pick for a fight you are losing.