McDonald's has some incredibly rigorous requirements and processes to ensure consistent qualities in their Sprite. I have no interest in propping up a giant harmful corporation, but their Sprite really is different.

Random video discussing this: https://youtu.be/ATEMvaOcWWc

The Food Theorist and many others have made videos discussing it as well. A few months ago I saw a particular one explaining the McDonald's process at length, but unfortunately I cannot remember enough details to find the same one again.

Side note: in typing this comment and trying to find relevant videos, I had to type "Sprite" an unreasonable number of times, and not once did my phone's keyboard get it right.

I don't know if this is true but I've read that, in their fountain sodas, McDonald's uses like 15% more flavoring syrup than what's recommended, so the sodas taste extra good