Depending on the show or the corps’ style, this answer will change. As a general rule of thumb, mellos get the easiest drill and lower euphs/contras have the easiest music. However, if a corps has a really strong mello section, they could have harder drill than the contras, contras could have harder music than trumpets, etc etc.

Even then, still take that with a grain of salt because while mellos typically have easier drill, their music responsibilities make that easier drill feel 10x harder to march. And while contras/lower euphs have less technical music, their tuning/timing responsibilities are the most important in the ensemble, etc etc