I'm voting for Danny. He has "almost" everything Ghostfreak has and more. Ghostfreak's only real advantages are his teleportation and instinctive action which can be overcome as he lacks any sensing capabilities and thus has no way of knowing where Danny is if the later goes invisible, true this can be argued the same way but Ghostfreak has no reliable way to put Danny down even with the surprise element as the ghost boy's regeneration and buffs such as reactive evolution, rage power and statistics amplification through ectoplasmic energy will close the gap in no time and put them on even footing which also includes speed as he was slower than a weaker Tucker Phantom and then became faster than a more powerful version of Tucker
Danny hard resists Ghostfreak's possession, meanwhile, Danny's possession has multiple layers and effects both the soul and consciousness, the same goes for his ectoplasmic energy attacks which he can apply the effect to as well and damage Ghostfreak on that level and overcome his regeneration. Forcefields are also a good method of blocking and reflecting Ghostfreak's ranged attacks and Fenton Thermos seals the deal if if my hits. Now you can argue with prior knowledge, Ghostfreak will just just spam teleporting but at that point Danny wouldn't be wasting much energy and then just reach Ghostfreak's level then overwhelm with amps. True, Ghostfreak gets buffed in the Darkness but Danny can generate literal light from his hands and light up the area.
I don't think the fight will last long enough for Ghostfreak to take advantage of the stamina difference (I'm assuming he has infinite for this fight) since Danny's stamina reached the point were he spends his nights rounding of ghosts back to back and should be at least comparable to his sister who spent the day fighting off ghosts with their father so Danny can get hours out of his ghost form if he uses his energy right and since he has knowledge, he'll know it's futile to try and waste a large amount of energy early on in the fight.
