Depends on what you ask because Audi has the Quattro, Mercedes has AMG and BMW copies everything. So I will be comparing Mercedes and Audi as BMW doesn’t really innovate.

AMG and Mercedes:

AMG is the first comercial car to use a “Hot V” configuration for the Turbos, which came because of F1, this “Hot V” configurations gave Mercedes 3 dominant championships in F1 *This is also used on all Mercedes-AMG Race Cars

Mercedes is the first company to mass produce a vehicle with Portal Axles (G-Class 4×4 Squared)

Audi has arguably the best AWD system on the market.

Audi has won most of the past 10 years at Le Mans with the R prototypes

Both companies have a big motorsport reputantion (DTM, F1, WEC)

So I would say in the power train point of view they are pretty equal, even tough Mercedes engines are known to be more reliable, on the power font, they are pretty similar (ex:RS7 and E63S)

It’s quite unfair to compare these three legendary car makers because they have their own legacy benz maintain luxury(Maybach series) while BMW is meant for high safety vehicle(5 series) and Audi has excellent performance in high speed cars (R8,R6)

So we can’t actually distinguish these three it depends on the need of buyer!.

Individually? I’d vote MB. If you bring in the engineering powerhouse that is the entirety of the Volkswagon-Audi family, Audi might have the edge. It is a close race though.

BMW is great, but I haven’t seen quite as much raw innovation. Maybe I just haven’t been exposed to it.