It is not possible to decide which could be the best app development company for a fitness app, yet you can check these factors for certain help.
Look for the company’s experience and the experience of their developers
Testimonials of previous clients or any feedback for that case
Portfolio of the company and history of developing such apps previously
Cost estimation of the app and post-launch support
Do not just go with one or two companies and decide upon. Refer to more than 5 or 6 and shortlist the best.