Garou Analysis
Main Win Conditions
While I don't believe, Garou has any actual win condition, that's not to say he doesn't have the means for an inconclusive and I'll explain why.
At the start, he's already going to have a massive AP advantage, effectively making any standard assault Dan does useless. Stack on his Enhance Senses, Analytical Prediction, Instinctive Reaction, Adaption, and Reactive Evolution, I see no easy way for Dan to hit him with anything that would be beneficial to his victory.
So effectively, Garou can just survive due to his superior statistics and by using his skill and abilities.
Supportive Abilities
While his regeneration isn't anything to screem at, it would still allow him to recover from basic attacks that could ignore durability like Ghost Rays as they are small beams that can hit on a molecular level. His resistances would make Dan's heat, electricity, and ice powers useless.
Dan Phantom Analysis
Main Win Conditions
A much as I'd like to say Dan just outlasts and to be honest, he eventually would but that would take an unquantifiable amount of time given Garou's stamina and by the fact that he wouldn't be using that much stamina given he'd have the statistics advantage.
With that said, Dan's
Ghost Physiology is his key to stalemating Garou. It provides him with regeneration beyond what Garou can overcome and infinite stamina so he'll be able to keep going forever. Add in his different intangibility and the fact that his body is made entirely of ectoplasm so passive radiation, and Garou's fighting style such as pressure points and durability negation are effectively useless.
Supportive Abilities
Dan's own Accelerated Development/Reactive Evolution will allow him to keep up with Garou for at least some time before being left behind. His own Portal Creation/Dimensional travel should stop all attemps at BFR, and not only do ghosts have resistance to empathy manipulation, Dan removed all of his emotions so he doesn't feel them and has Supernatural Willpower, so Garou's fear manipulation shouldn't be an issue.
My Vote
As I said, I'm just going to vote Inconclusive.
Both either resist or can easily avoid each others abilities. Garou's superior fighting skill and specific abilities will be his edge, meanwhile, Dan's Ghost Physiology will be his edge. The fight will basically get nowhere so, "The Ultimate Enemy" and the "Absolute Evil" just chill out and reflect on their stupid Ls they took by the protagonists.