What is the best app for solving maths?

I'm curious, could you please tell me what application you would recommend for effectively solving mathematical problems? Is there a specific one that stands out in terms of its user-friendliness, accuracy, and comprehensiveness in covering various mathematical concepts? I'm looking for an app that can assist me in tackling challenges from basic arithmetic to more complex topics like algebra, calculus, and even geometry. Your insight would be greatly appreciated.