The hardest repair on a car can vary depending on the make and model, but some commonly cited difficult repairs include:

1. Transmission Repair or Replacement: This involves removing the entire transmission from the car, which is a labor-intensive and complex task due to its intricate components and the need for precision.
2. Engine Overhaul or Replacement: Rebuilding or replacing an engine requires deep mechanical knowledge and precision. It involves disassembling numerous parts, diagnosing issues, and then reassembling everything correctly.
3. Timing Belt Replacement: This task can be challenging because it involves removing several components to access the timing belt. If not done correctly, it can lead to severe engine damage.
4. Head Gasket Replacement: Replacing a blown head gasket requires significant disassembly of the engine, which is time-consuming and requires a high level of skill to ensure proper reassembly.
5. Electrical System Repairs: Modern cars have complex electrical systems, and diagnosing electrical problems can be very challenging. Repairing or replacing wiring harnesses, control modules, or sensors can be intricate and time-consuming.
6. Suspension Work: Repairs involving the suspension system, like replacing the control arms, struts, or shocks, can be difficult because they require precise alignment and handling of heavy components.
7. Body and Frame Repair: Fixing significant body or frame damage, often due to an accident, involves specialized skills and equipment. It can be very labor-intensive and requires precise measurements to ensure the vehicle is structurally sound.

Each of these repairs requires a combination of skill, experience, and sometimes special tools, making them among the most challenging tasks for mechanics.

The most commonly repaired item on a vehicle is often the brake system. This includes components such as:

1. Brake Pads: These wear out regularly and need to be replaced periodically to ensure proper braking performance and safety.
2. Brake Rotors: Often replaced in conjunction with brake pads, rotors can become warped or worn over time.
3. Brake Fluid: Needs to be replaced or flushed to maintain the hydraulic pressure needed for effective braking.

Brakes are crucial for vehicle safety, so their maintenance and repair are frequent and essential.