About B Spheres
They generally lean toward improving defense capabilities.
Keywords for the abbreviations below are referenced at the bottom
What is best?
Every character is different, but there are some general rules
ATK spheres should be used on hybrid units that already weigh heavily toward the attacker role
BRV gains help tanks with taunts avoid breaks after ability use
Lowering party BRV damage & HP damage can help feed into the shortcomings of a character's defensive kit
All B spheres
The thing to look for with these spheres is whether your character can inflict the Lock debuff, or can cover your allies from attacks. Take HP Damage is a frequent trigger, and defensive characters that don't Lock/Cover will be forced to choose between Celes, Gabranth, Kimahri, or Warrior of Light spheres.
Keywords used under character images:
crit - when dealing a critical hit
take DMG - when taking HP damage (self)
on evade - after dodging an attack
self target - when attacking an enemy that is targeting that character