Personally, I think the fireball is the most effective, particularly in wilderness environments, when fighting in large subterranean openings, or when dealing with a lot of enemies who can't reach the caster/party. The range is outrageous, making it one of those spells that never really loses its value. I had a single Pit Fiend do tremendous damage to a group of PCs with this spell (especially since the Pit Fiend had the ability to cast it as a quickened action each round. Teleport, cast! Cast, teleport! HAHAHAHAHAHAAAA!!!).
However, for sheer style, use in long, enclosed places (like halls and corridors), and to give great "pre-descriptions," lightning bolt is the way to go. Each time the DM describes how their hair raises up on their heads and neck, how they smell ozone, and how the air seems to crackle, PCs will groan or yelp in anxiety. It's also a useful water attack if casted from above (I treat it as a fireball with regards to area of effect).