Need that hasn't been properly cleaned in a decade taste.

Fountain soda typically uses a higher ratio of syrup to water. This is in anticipation of some ice melt.

It's mixed at the tap. At a soda fountain thw water, CO2, and syrup are stored separately and mixed at the tap. Flavor difference is likely the result of a different ratio.

The seltzer water and soda syrup are mixed when it's pouring into your cup. If the machine is properly maintained, and the syrup hasn't "expired", it should be the freshest soda can get.

Source: Years of working food service.

Its not, canned is best, followed by bottled. Fountain is the bottom drink of last resort.

Disagree, glass bottled is best, followed by canned, then plastic bottled.

Fountain soda ratios are always way off.

I find bottled/canned soda has way too much carbonation. fountain drinks are usually better for that alone.

This is specific to McDonalds, but the principles apply: https://youtu.be/4VETXsKd0Z8