Dr.Linus Pauling, winner of 2 Nobel prizes, believed that Vitamin C was the best supplement for stiff arteries. It is said that he used to take 12 grams of Vitamin C every day at one point of time. This does not mean that we should all take that much, but that Dr.Pauling believed that higher-than-conventional doses of Vitamin C gave great results and that each person should take the amount of Vitamin C that their own system can tolerate. Dr.Andrew Saul has a great page on this and you can find it here.
Another supplement that has research studies backing it to be great for arterial health is L-Arginine, the most active ingredient in red wine.