BMW Z8?
Spawned the Z3 and Z4, proved there was a market in the modern world for a high powered, 2 seater luxury convertible sports car.
I would suggest the SLK ITSELF would not have existed without it.

Saying that one feature being imitated makes a car "influential" seems unreasonable, but if you want to play that game-the first production convertible hard top was the Ford Skyliner in the fifties, and the first to repopularize it in modern times was the Mitsubishi GTO in 1995.
I therefore claim that the GTO is an "influential" car.

Don't get me wrong-I love these cars, but they were NOT regarded as any kind of success.
The SLK was widely criticized for looking bad, handling sloppily and being too heavy.
The steering is woolly and unresponsive, and the brakes fade.

I love mine anyway.