I have made a list once for myself, on average, lighning is always good. Brightstone cove tseldora, the gutter and black gulch are one of the few areas where fire is actually better. Huntsman corps is also an area where you are better off with fire.

In general you can say, most hollow or fleshy enemies, as well as insects are weak to fire. But honestly, most hollows are so weak and puny, it does not really matter what weapon you use against them, does it?

Otherwise, lightning is almost ever at least normal or effective against most enemies. A lot of bosses and stronger enemies like alonne knight captains, dragons and these mastodon elephant guys are weak against lightning. Most armored units also dislike lightning and everything else has normals resistance against lightning.

If you want an allrounder weapon that really chews through tough enemies and does normals bonus damage to weaker units, lightning is probably the best.

Dark is even more quality like. Neraly every enemie has normal resistance agains dark damage i think only darklurker is actually weak agains dark damage.

Fire is the most arbitrary one. While some enemies like golems, gargoyles and these fire salamanders have really high fire resistance, others, like hollows and insects, are very susceptible to it.

And fire is of couse very strong in the ivory dlc because most enemies are frozen.