When something is ne plus ultra, it's the very best or the highest point that can be reached. Making it to the summit of Mt. Everest would be a mountain climber's ne plus ultra.
The Latin phrase ne plus ultra literally means "no more beyond," and it's famously said to have been inscribed on the Pillars of Hercules at either side of the Strait of Gibralter, in part as a warning to sailors not to travel beyond the edge of the known world. Today it's used to mean "perfect," or a flawless example of something. For you, the ne plus ultra of cars might be a vintage Mercedes.