Since the OP can vote, I'm just going to put my thoughts below.
Majin Buu's Advantages/Win Conditions
Buu starts off with a massive power advantage which will allow him to easily blow away or tare through Danny, tie that in with his fighting style that allowed him to face off against the likes of Goku, Buu definitely has the skill advantage as well.
While Buu's candy beam won't keep Danny incapacitated since ghosts can reshape/reconstruct their physical form, Buu still has his absorption which could work as Danny's possession won't override it when he's absorbed. This is because season 1 Danny lacks resistance to corruption and as we saw in early season 2, it is directly stated that Vlad's evil was so great it overwhelmed Danny and turned him into Dan.
Given how evil Buu is, if he absorbs Danny, Danny's will won't be able to overcome Buu's evil. And if Danny tries to overshadow Buu first, it will result in an inconclusive because Buu's evil will turn Danny into a different person entirely. Finally, Buu's regeneration will allow him to withstand anything that isn't completely molecular vaporization, and his stamina isn't bad either.
Buu has other small advantages like IT, and the massive range advantage but those aren't game breaking given Buu doesn't BFR and Danny has was of avoiding Buu's large scale attacks.
Danny Phantom's Advantages/Win Conditions
While Danny starts out at a massive disadvantage in terms of power his reactive evolution is shown capable of buffing him up to his opponents level (within reason), and Buu is 4-B while Danny's reactive evolution has allowed him to reach the
Fright Knight's level which is at least 6-C, possibly 4-A. So Buu is within Danny's evolution range which means Danny can close the gap fairly quickly, making them around equal.
While Danny may not be as skilled as Buu, he has his ghost abilities aka
Ghost Physiology (Danny Phantom), this grants him a similar body manipulation like Buu so he can avoid small attacks by opening holes in himself or just turn intangible to avoid large ones. Boo has no counter for Danny's invisibility as Danny doesn't have ki.
Ectoplasmic energy hits on a molecular level so if Danny uses a wide enough energy blast, he can bypass Boo's durability and overcome his regeneration. Meanwhile, Buu lacks the ability to deal actual pain to Danny as ghost can't be harmed by normal stuff (He can still damage Danny's body and force him to regenerate, he just won't cause him pain). Danny's regeneration will allow him to recover from Buu's attacks at the cost of stamina, however Danny is stated to be capable of hunting and fight ghosts throughout the night so he can go for hours normally so he won't get exhausted to fast.
Danny also has attack reflection shields so he can reflect Buu's energy attacks back at him or just block them.
My Vote
I'm going to vote Inconclusive in my opinion.
Boo starts out stronger, is more skilled and unpredictable, can make Danny exhaust himself if he applies constant pressure throughout the fight, and if he lands an absorption it's a win. Now Danny can evolve to Boo's level and has abilities like invisibility and intangibility are great (though he doesn't spam them), plus other means of avoiding or reflecting Boo's attacks back at him so he could potentially reflect the candy beam.
However Danny doesn't know if Boo is a ghost or not and since Danny knows he's already in a fight and sees Boo's abilities, he'd think he's a ghost so his Ghost Sense has no reason to trigger and once he sees Boo is immune to the Fenton Thermos he might go for a possession which would result in a hard inconclusive as I explained in the Boo segment of my analysis.
In the end, I see Boo having a better chance of victory at the begging but due to Danny's abilities, it gets evened out fast and both have a chance because their have been times when Danny has outright admitted he should have used his intangibility but didn't so Boo's absorption is possible but so is Danny destroying Boo on a molecular level.
