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.
One interesting crossover could be Ghost Rider teaming up with Batman. Imagine the contrast between Batman's detective skills and Ghost Rider's supernatural powers. They could face a villain who has both a criminal underworld connection and a dark magic aspect. Another could be Ghost Rider and Wonder Woman. Her strength and his hellish powers would make for an epic battle against a common enemy like Ares, who might be corrupted further by demonic forces.
One idea could be that Ghost Rider somehow gets transported to the DC universe during a magical mishap. He teams up with Constantine to battle a demon that's threatening both the DC world and his own. Another plot could involve Ghost Rider's hellfire having an unexpected reaction with Kryptonite, creating a new type of weapon against Superman's enemies. Or perhaps he meets Raven and they combine their dark powers to fight an ancient evil that neither could defeat alone.
To balance the powers, in a Ghost Rider x DC Fanfiction, you could limit Ghost Rider's access to his full hellish powers at times. Maybe he's in a weakened state when he first enters the DC universe. For DC characters, you could give them a handicap based on the situation. For example, if it's Superman, make it so kryptonite is involved in the story in a way that levels the playing field.
The story of Ghost Rider involves a person who becomes possessed by a supernatural spirit and gains supernatural powers to seek vengeance against evil. It's a tale of redemption and justice in a dark and supernatural world.
There are probably some 'Harry is the Ghost Rider' fanfictions floating around. To find them, you could also try joining relevant fan groups on social media platforms. Fans in these groups might be able to recommend some great ones they've come across. Additionally, when you do find some fanfictions with this concept, pay attention to the reviews and ratings. A highly rated one is more likely to be a good read, but don't be afraid to try out some less popular ones too as they might have a unique take on the idea of Harry being the Ghost Rider.
The Ghost Rider comics have a history that dates back several decades. They originated in the 70s and have had various iterations and storylines over the years.