What acoustic guitar strings professional use?

Unless they're endorsed it usually says. I like Dean Markley ones, but as long as they last and stay in tune you can be using the cheapest and it won't make much difference after the first week or so sound wise.

Answer

Soloist/bands use whatever they are paid to use. Session musicians tend to be more selective; they chase their signature sound. And strings that sound good on one setup could sound flat and dull on another; good on an ibi, sad on a tele. Time and choice will find the best for you.