Ugh, one of my favorite character losing to a character i also like
Observation haki precognition is >>> any analytical prediction that can be achieved by skill, as we can see in Luffy being utterly helpless against Katakuri, who could see an absolute future that can only be circumvented by canceling it with your own precog.
Not to mention in one piece you can actually become a top tier without a Devil fruit, but without haki? Even with extreme skill, though luck.
Stamina, while both are very impressive, One Piece is infamous for stuff like fights lasting for weeks while characters are heavily injured, so i'd give the edge to Zoro.
Actual skill goes to Garou, since without Haki i really don't see Zoro keeping up at all ( Garou can insta-copy martial arts from Grandmasters after seeing it once, like when he saw Bomb perform a single kick, and came up with the entire rest of the martial art himself, including the combo moves that require 2 great masters to perform, has low level pressure points that make characters lose stamina faster, IR that allows him to dodge multiple shots that can ricochet in a narrow dark alleyway from multiple angles, can redirect multiple attacks with one-shot potential if they landed with WSRF, Defeated world champion martial artists/stomped multiple dojos before he started hunting heroes, and this is not even going into Webcomic stuff, like awakened Garou. )
Garou's RE is actually strong here, since half monster Garou is growing stronger by the moment and can go from getting all his ribs broken from a punch from Darkshine to matching him in raw strength in a few exchanges of blow, it can also give him resistances, but there's nothing like Haki in OPM, so forget it. Speed increase also happens constantly, and can eventually lead to a blitz, but i don't know if it would be allowed here.
But thanks to Haki, Zoro has dura neg + precog that is >> analytical prediction and very big sword shockwaves, he can also tank damage from Garou for a good while using it defensively and endure it for a lot. So i'd say Zoro takes it here high diff, as all he truly needs is a well landed slash at any point of the battle to end it, and his shockwaves are BIG, and he can also do fancy stuff like Tatsumaki.
Btw, why are we using feats for analytical prediction Garou did in his awakened fight against Saitama in the webcomic, if we decomposited them?