Most spheres act as the most minor improvement to a character you can make, and will have almost no impact on the results of a fight, not even shaving off a single turn from turn count.
Spheres are just to make your favorites look better, though in certain cases, as noted below, can expand role coverage
All characters want ATK or BRV DMG up, both are very effective, and there are far too many options to recommend specifics as most players won't have access to them.
Gladio/Snow if character can Lock
Gabranth/Celes if character cannot
Paladin Cecil for Encouragement armor holders
Garnet for attackers that can heal
Alphinaud / Maria otherwise
Any ATK sphere that the can character can trigger is good, no others need to be considered
ATK spheres for attackers, Seymour/other
Paine if character can hit weakness
Quistis for other supports
Faris/Fujin for non-debuffers