Ok here I go crushing arguments, hopes and dreams. Sorry Obito haters, this isn't your day.
Guy fighting Madara is not an argument, period. Madara wasn't taking Guy seriously, barely fighting back for that matter. He was excited and drew out the fight, because fighting the 8'th gate of death is literally a once in a lifetime opportunity.
Guy had outside help. First Lee to throw Minato's kunai, then Minato teleporting away Maara's TSB's, which would've insta-gibbed Guy otherwise. Plus Kakashi teleporting away Madara's TSB barrier so Guy could get a hit in.
The Night Guy would kill Obito....if Guy goes for the head. We saw Madara's healing factor from only having an arm, and a face worth of Hashirama's cells. Obito is literally made out of half of the stuff, so any damage Guy does manage to do to Obito will be all for naught short of pulverizing his head.
This is even assuming Obito would let Guy charge up the Night Guy. Unlike Madara, Obito doesn't have the Moral Compass/ Honor Warrior code Madara has, he'll just straight up kill Guy.
This is how I see the battle going. Guy throws his first punch that pushes Obito back, now realizing the power of his opponent realizes he can't pull his punches. In the time Guy stalls as he acclimates to the pain of the 8'th gate, Obito sends a TSB orb, or spike impaling Guy's Torso, or straight up blowing it off.
If by some miracle Guy survives long enough to start charging the Night Guy, Obito creates the sword of Nunoboko, atomizes Guy while he's charging his attack.
Seriously, unless Obio just stands there, Guy has no chance. Obito 10/10, this is a stomp fight.