Most of my local McDonald's have switched to that new form of fountain. You know the ones. With the touch screens and lots of syrup flavor options. I'm not convinced it tastes "better" than other forms of Coke anymore. It tastes fine though.
Anyway, Mexican coke (glass bottle) is the best. If it uses sugar. I read that they might switch to corn syrup for those, but so far they still seem to contain sugar anytime I read the ingredients.
After that, cans. Followed by plastic bottles. Followed by other varieties. Fountain is too hit or miss, as if they are not calibrated right or something else in the chain isn't right it could taste foul.
The true acid test for me is the slight bitter flavor coke has when it's properly prepared. Some coke sources don't have this right for whatever reason. But it's always there in 20oz bottles and glass bottles from my experience.
I think one of the keys for cans/plastic bottles is freshness. If it has been sitting too long it might be leeching some kind of flavors from the container. It's the same reason I don't like drinking coffee from a stainless steel travel container. It changes the flavor a lot.