Overall Truck and Motocross bike seems to be the best vehicle in Hill Climb Racing. To score High-Score one can take a full upgraded truck and drive in highway very easily for at least 13000m where you can get up-to 2500000 coins (it may take about 15 minutes of gameplay). With motocross bike in moon one can get 100000 easily by airtime and flips as the gravity is less.
But for some levels other vehicles prove to be best. For example in Xmas and Forest levels Hippie van will be best as it passes through the trees (https://youtu.be/x1LYHt074tk). For levels with liquid material in between like Mudpool, Volcano, Nuclear Plant the vehicle Hovercraft will be the best. Big Finger will offer some protection over flips as it has roll cage. Kiddie Express, once the extra weight is detached, will help reach far ranges with its fuel capacity.
Race car will be the fastest. Moon lander's thrust will be useful to avoid deep falls. One wheeler, Tourist bus and Sleigh doesn't help much.
The levels Northpole and Highway will be good for almost all vehicles.
