NGK is my first choice when changing brands. But it still very much depends on the specific engine. Generally, the OEM plug tends to offer the best performance balance in most cases. While plug makers would have you believe differently, the engineers do build ignition systems around particular plugs. For example, nothing betters the OEM French-made Volvo plugs in some I-5 Volvo engines, even though it is a standard metallurgy design only costing a couple dollars.
VWs are using more NGKs as OEM, so that result doesn't necessarily come as a big surprise. But if the Merc came OEM with a Bosch FR8-DPP33, that would be my first pick unless there was reported proof that another plug performed better in that particular engine . . . especially when you're buying 16 of them.