Well a lot of people say Kitsuchi so imma go Zabuza.
Firstly, Kitsuchi's advantage lies in a superior element. Other than that...I'd.give most of the edges to Zabuza.
From what we've seen, Kitsuchi's two main attacks with Doton are making sandwich trappings with the earth, and applying rock fists.
His trapping jutsu seems to take far too much time to be used on Zabuza...as both times we've seen it, it took tremendous time against a idle opponent. Now he could opt for a smaller sandwich, but it seems as though mobility allows one to more readily escape it.
Even worse for Kitsuchi is that he must co.tend with Silent Killing. Zabuza is a strong opponent in open combat; however, he would be one to employ Hidden Mist meaning that on top of his semi-slow earth mountain jutsu, he also has to track and use it against an opponent who practices silent moving as well as doing so in low visibility.
Now in open combat and aside from silent killing/Rock Mountain jutsu....it becomes a closer fight. Both are about equal in physical strength. Kitsuchi is a taijutsu/close quarters specialist (this was his division)....and his augmented rock fist puts him toe to toe with Zabuza.
Like wise, Zabuza is a power house. He has the strength to throw around the Executioner Blade like its a play thing. It took an arm augmented Suigetsu to pull off half of what Zabuza does with two hands. Adding to his physical strength comes two more assets. His Kenjutsu talents are among the best....making it an effective counter/parallel to Kitsuchi's rock fist taijutsu. Additionally his stamina/endurance is on an insane level. A broken spiritually Zabuza was just able to keep Kakashi on edge. And the Zabuza driven with emotion could charge through an entire personal army without arms to kill the Mob Boss. Taking a shit load of spears.
While they are hardly contenders, Zabuza still has the water dragon and water vortex jutsu to keep Kitsuchi on defense if the situation calls for it...though they are ineffective.
Combining hidden mist, silent killing and excellent Kenjutsu, I'd give Zabuza the edge.
Hidden mist lowers the use of the Earth Mountain sandwich. His silent killing gives him an edge over Kitsuchi in preparedness so I'd say that is what pushes their probably equal CQB skills in Zabuza's favor.
So I'd give Zabuza 6 or 7 out of ten times.