One idea could be that Danny and the Winchester brothers team up to fight a ghost that is causing chaos in both the Ghost Zone and the human world. Maybe it's a powerful entity that has the ability to cross between dimensions easily.
Definitely Danny and Dean. Dean is all about hunting monsters and his initial reaction to Danny, a half - ghost, would be very interesting. He might be skeptical at first but then see Danny's potential in fighting the supernatural threats that are a bit out of his usual realm.
For the crossover, a good plot could be that a new type of ghost appears that is immune to Danny's ghostly powers. The Ghost Hunt team, which has encountered a similar situation before, offers their expertise. They introduce Danny to a special type of ectoplasm - based weapon that can harm this new ghost. Danny then teaches them some of his combat moves as a half - ghost to better fight the ghost.
One idea could be that the Totally Spies are sent on a mission to investigate some strange paranormal activities. They then discover Danny Phantom's world and team up with him to stop an evil force that is using both magic and technology to take over the world. Danny could teach them about the Ghost Zone and they could share their spy skills with Danny.
A good Danny Phantom crossover fanfic should have a well - developed plot that combines the two universes in a logical way. For example, if it's a crossover with Teen Titans, the reasons for Danny and the Titans to meet and work together should make sense.
One idea could be that Danny's ghost powers are mistaken for some sort of demonic or otherworldly power by the Winchesters. They might initially try to hunt him down, but then realize he's not a threat and they team up to fight a greater evil that is related to both the ghost world and the supernatural creatures they usually deal with.
Check out blogs dedicated to Danny Phantom. Some bloggers review or recommend fanfics. They usually give a brief summary of what the fanfic is about and what makes it good, which can help you decide if it's the best Danny Phantom crossover fanfic for you.
In a Danny Phantom and Justice League crossover fanfic, Danny could team up with Superman. Maybe Danny uses his ghost powers to help Superman when he's facing a powerful ghost - like enemy that the Justice League can't handle with their normal tactics. It would be a really cool combination of the two universes.
There could be a fanfic where Danny and Ember accidentally swap powers. This leads to a series of comical and also dangerous situations. Danny has to figure out how to control Ember's power of influencing people through music while Ember struggles with Danny's more combat - oriented powers. It could be a story full of self - discovery for both of them.
I like the idea of a Danny Phantom and Ben 10 crossover. Ben has the Omnitrix which gives him access to different alien forms, and Danny has his ghost powers. They could either be rivals at first, each thinking their powers are better, or they could quickly team up to face a threat that requires both their unique abilities. It would be a fun clash of two very different power - based universes.