^I once saw a Ford Ka from Mexico.
Anyway, my nominations:
Ford
Taurus
Ka
Mustang
Pretty much every fast Ford ever (exclude Gen-1 Focus RS).
Mercedes-Benz
300SL
S-Klasse
190E 2.5 16
CLS-Klasse
Nissan/Datsun
240Z
Skyline GT-R R32
Z32 300ZX
GTR
In reality, I think Ford probably has done the most signicant cars of any company I know (I think I've missed some French and Italian companies), including the too-often-cited Model T. They have two fairly independent and huge sections (Ford America, ford Europe) and both have produced some important and lovely cars. The Taurus was the first mass-market aerodynamic car for America, and was unusually sporty; the Ka is great to drive and extremely clever (reminds me of the old Mini); the Mustang is superpopular, deserved or not; and the 'Fast Fords' part goes without saying.
Mercedes is very advanced and, excluding the 90's, a very characterful company. Their 300SL was mad; the S-Klasse is always from the future (see: Top Gear); the quick 190E is a longer M3 with more interesting backstory; the CLS is unbelievably gorgeous. And, all the old, superdignified sedans don't hurt either.
Nissan is definitely more Japanese in philosophy (Simplify and add buttons?), but they still accidentally produce something epic every once in a while. I admit, my Altima biases me towards this choice, but let me explain myself. The 240Z brought beauty to the masses, the R32 GT-R slaughtered everything while retaining rather a lot of character; the admittedly unpopular Z32 300ZX prompts inquiries of (That a Lambo?); the new, handbuilt GTR resumes the tradition of slaughtering mercilessly. I don't personally think the S13 Silvia is a properly great car, but it has enough status to be worthy of mention.