All them, but the legs and buttocks do most of the work in cycling. Here are the groups, (quadricep) this straightens the leg,(hamstrings)this muscle bends the leg,(gluteals) these are your buttocks and the(gastrocnemis and soleus) are your calf muscles. If you're riding well hunched over you'll bring your abdominals and back muscles into play to some degree as well. Riding out of the saddle will engage arms and shoulders too.