What is the best and cheapest load to test a variable frequency drive at full load?
The cheapest and easiest test is to couple the VFD motor to a similar motor connected to mains power. Using the VFD motor to drive the mains-connected motor about 3% above synchronous speed will present full load to the VFD motor. That will provide a reasonable test over a range of loads from zero to at least 150% of rated load. The load is easily controlled by varying the speed between synchronous speed and about 5% above synchronous speed. This method provides no capability for testing the VFD at any operating frequency other than mains frequency and slightly above.

A centrifugal pump is a good test load, but the pump curve controls the load at any given speed.

The best test load ultimately depends on how thoroughly you want to test the VFD. A really thorough test requires a dynamometer.