I think the Ghost Rider would be the most interesting. His otherworldly powers and his tortured soul make him a very unique character. In the Underworld setting, his hellfire and penance stare would be extremely effective against the vampires and lycans. Also, seeing how he interacts with the different factions in the Underworld, whether he's feared or respected, would be really cool to explore.
Selene from Underworld could be a great focus. She's a strong and complex character. When she meets the Ghost Rider, it could be a clash of two very different but equally powerful beings. She has her own vendettas and goals in the Underworld, and now she has to deal with this strange new entity. Her skills as a vampire warrior and her knowledge of the Underworld could be put to the test when faced with the Ghost Rider's powers.
Artemis would also be a great character to focus on. Her archery skills combined with Ghost Rider's hellfire could create some really cool combat scenes. Plus, her background and her own internal struggles could be contrasted with Ghost Rider's own tortured existence.
Definitely Constantine. Constantine is a master of the occult in the DC universe, and Ghost Rider has his own connection to the supernatural. Their combined knowledge and powers could lead to some really cool adventures. They could face off against demons and dark magic users that neither could handle alone.
Definitely Naruto and the Ghost Rider. Naruto's energetic and positive nature would contrast so much with the dark and brooding Ghost Rider. Their conversations would be really interesting, like Naruto trying to understand the Rider's concept of vengeance while the Rider is trying to figure out Naruto's world of ninjas and chakra.
Definitely Ghost Rider and Thor. Thor is a god, and Ghost Rider has connections to the underworld. Their clash of powers and different mythologies would be fascinating.
Alice from Twilight could be a fascinating character to focus on. Her ability to see the future could play an interesting role in a crossover. She could either help or hinder the Ghost Adventures crew in their investigations. Maybe she sees something in the future that involves the ghosts and tries to prevent or encourage the crew's actions.
Raphael could be really interesting. In the 2003 version, he is hot - tempered and tough. In the 2012 version, he also has a strong personality. Seeing how he reacts to his counterpart and how they might either clash or bond could be a great focus. Maybe they have a friendly rivalry as they compare their fighting skills.
Ross could be really interesting. Since he's an archaeologist, in a crossover with a show like 'Indiana Jones' (in a fanfiction sense), he could go on an adventure with Indiana Jones. Ross would bring his academic knowledge while Indy would have the practical experience. Their interactions would be filled with Ross trying to over - analyze everything and Indy just wanting to get the job done.
Chiaki Nanami could be an excellent choice. She is the Ultimate Gamer, so if you cross her with a digital or virtual world, there are so many possibilities. She could use her gaming skills to navigate through a new digital realm, like a Matrix - like world. Her calm and intelligent nature would also make her a great character to lead the group in a crossover situation. She could figure out the rules of the new world quickly and help the others adapt.
Definitely Damon Salvatore. He's the bad - boy vampire with a heart of gold (sometimes). In a crossover, he'd be a wild card. He has his own sense of justice and loyalty, but also a devil - may - care attitude. He could get into all sorts of trouble in the world of the Originals. His sarcastic remarks and his ability to charm his way out of (or into) situations would make for some great scenes. He might also form an unlikely friendship or rivalry with one of the Originals, like Kol perhaps. His backstory and his experiences in 'The Vampire Diaries' would contrast and collide in interesting ways with the Originals' world.
Naruto himself would be very interesting. His energetic and determined personality would make for great adventures in the GTA world. He would probably approach the chaos and crime of GTA with his usual enthusiasm, trying to bring justice in his own ninja - way. His ability to make friends easily could also lead to some interesting alliances with GTA characters.