Although tank doesn't look like the best vehicle, I found it very good to use in mountain level when all attributes are maxed.
gaming.stackexchange.com/users/62133/Micer
By my experience, the best vehicle is the 4x4 Diesel.
It allows to go upwards easily and is difficult to turn down.
Also, you will have enought fuel to think the difficult jumps (or try again some of them) due to is long duration fuel.
gaming.stackexchange.com/users/43105/SysDragon
I don't have every vehicle unlocked, but I got pretty far with monster truck and super diesel 4x4.
I don't think there is one vehicle that is the best, but a vehicle type.
mountain is very well, mounatainy.
so you want a vehicle with good climbing ability, but not too much power that you just flip over when trying to climb.
I would guess the dune buggy and hovercraft would work very well here as their descriptions say they are the best climbers (but again, I don't have them unlocked)
gaming.stackexchange.com/users/47011/Rapitor
I unlocked all the vehicles and stages.
According to me, the diesel 4×4 and race car are the best, as you need speed to cross this stage.
You can try the race car or dragster.
gaming.stackexchange.com/users/65517/prateek
My current best is 1850m with the Hovercraft (stability 18/20), so you could probably do more.
I touch pretty much only with the back of the car, advatage is that oyu can vertically upwards, as long as something is touching the ground.
Plus you can't get stuck in a small dip in the terrain.
gaming.stackexchange.com/users/75901/user75901
The Mountain stage works best with vehicles that are good at making short, steep climbs, and are resilient to flipping.
Generally, that means the Tank, and in my experience this stage is no exception.
My best run has by far been with the Tank, at 2114 meters.
Next up is Monster Truck at only 1363 meters.
The treads on the tank also handle the large number of sharp points very well, when other vehicles get stuck with a tire on either side of the point.
gaming.stackexchange.com/users/19748/Chris Marasti-Georg
Which vehicle is more appropriate for the "Mountain" stage of  Hill Climb Racing ?
Which vehicle is more appropriate for the mountain stage of Hill Climb Racing?
gaming.stackexchange.com
<android><hill-climb-racing>
gaming.stackexchange.com/questions/141627
gaming.stackexchange.com/users/15861/B Faley
Tank is good in mountain level, while the diesel 4x4 and monster truck give you speed to cross this stage. There are few vehicles that are proper for the mountain stage, such as the tank and the 4x4 diesel.
Players suggesting vehicles.