About C Spheres
They generally lean toward granting HP gains or BRV battery.
Keywords for the abbreviations below are referenced at the bottom
What is best?
Every character is different, but there are some general rules
Giving the party an aura with Selphie is clearly rare, just ensure you can trigger it
While Porom & Rosa's spheres are very rare, their effect is very minor. It's best to give it to a non-healer or a healer that is not as reliable at healing, to make them more reliable.
ATK C spheres
Clearly a rare sphere, it is the best sphere possible for an attacker that has HP Regen or burst healing.
Aura C spheres
These spheres are split. ATK spheres are among the best in the game. If no ATK aura, it's generally better than nothing!
Healing C spheres
Only edge cases may dictate the use of these spheres. Often used on supports that lack healing, lack adequate healing, or attackers that plan to solo difficult stages for fun.
Battery C spheres
Paladin Cecil's sphere or Refined (RF) sphere are the undisputed best for anyone with an Encouragement Keystone passive from their HG armor. Not only is it best, but it is the biggest effect that a sphere can have for a character. The rest of these are filler, occasionally useful for specific characters.
Keywords used under character images:
broken enemy - when attacking a broken target or breaking them
buffing - when granting a buff
self healing - when healing your own HP