Maybe I'm biased, but I vote for Professor Stokes as the most adept at handling supernatural emergencies.
Mrs. Johnson would be a very close second. Â Her common sense kept her grounded while the rest of the family was often caught up in some sort of paranormal mess. Â If ever she encountered something out of the ordinary, (e.g. Quentin's ghost) her reaction was always believable and never over the top. Â Harry wasn't on the show for very long and wasn't really exposed to the amount of weirdness that the other characters faced. Â Julia did see a lot during her extended stay at the big house, and at times rose admirably to confront whatever bizarre force had entered the Collins' midst (e.g. Nicholas); other times, she would go to pieces emotionally, fretting and crying to Barnabas.
Stokes knew the most about the occult, and his background proved invaluable on many occasions. Â He was intelligent and brave, able to keep his cool and refusing to be intimidated by the unknown. Â When the other characters were confused or frightened, Stokes always seemed to know what to do. Â Of the four, he is the only one who doesn't live at Collinwood, and I will admit that that probably gives him an advantage. Â Nevertheless, I still think he's the most capable candidate.
ProfStokes