The best overall "hand-held" commercial string trimmer is the Husqvarna 128LD; with a straight shaft, and a powerful 28 cc gas-powered engine, it is durable, lightweight, and affordable for most commercial applications.
As a runner-up to commercial jobs that include heavy-duty weeds or brush, we're big fans of the TAZZ 22" High-Wheeled Gas-Powered Trimmer.