What is the best tool to tighten a rigid 1-inch conduit coupling to a threaded conduit?

The best tool for tightening a rigid 1-inch conduit coupling to a threaded conduit is a monkey wrench. This type of wrench is specifically designed to handle larger, heavy-duty tasks, making it ideal for tightening fittings like conduit couplings, which can require significant torque. The jaws of a monkey wrench are adjustable and can grip around the larger diameters typically found in conduit fittings, providing the necessary leverage to ensure a secure connection.

Other tools, while they may be used for various tasks, are not as optimal for this specific application. An adjustable wrench, while versatile, may not provide the same level of grip and torque as a monkey wrench for larger sizes. Adjustable pliers are generally not designed for tight fittings and can slip, potentially damaging the threads. A crescent wrench, similar to the adjustable wrench, can work in certain situations, but it often lacks the versatility and strength needed for connecting heavy-duty conduit fittings properly.