Luffy would be the person most vulnerable to lightning attacks, as rubber, being a resistor, cannot channel a high concentration of electricity away like metals can. Fortunately for Luffy, this means that a lightning strike would rarely ever hit him, and if he were struck, it's unlikely it could kill him, as the electricity would not have a chance to pass through his internal organs, but would cause severe damage to the location of the strike. High voltages and lightning will usually just melt rubber.
Funnily enough, the character most logically capable of facing Enel would be Gan Fall, as though lightning would be more likely to hit him, any strike would be channeled by his metal suit of armor around his body, into the ground, never passing through any internal organs, preventing damage.
See: Faraday Cages; https://en.wikipedia.org/wiki/Faraday_cage
If the world of One Piece were governed by Earthly physics, Enel would be easily beatable by a knight in (shining) armor. But for us to suspend disbelief by believing in all the crazy stuff that happens in One Piece-Land, I assume we must also accept this.