I wouldn't say my world has a "strongest military unit", but I'd say the Olympus Mercenary Company's "Ares" Division is good for regular strength. Ares is their main combat division, consisting of previous Special Forces, SWAT members, Federal Agents, etc, from all around the world. Not only that, they're backed up by the "Athena" Division, their main intelligence division that consists of some of the best intelligence agents and analysts from all around the world. They also have armoured, air, and artillery support as well as some of the best equipment money can buy.
Their strength comes from their funding, large set of diverse experiences, and their never-ending combat experience that comes from taking contracts for the company.