Diet Pepsi tastes much more like Pepsi compared to Diet Coke/Coke. Pepsi Max could easily be second though depending on my mood.
I've convinced people with blind taste tests, but not much I can do over the internet.
Coke is too sweet and makes my teeth feel weird which is why most of the Pepsi products go above the Cokes.
I don't find Pepsi as bad as Coke with that. Coke leaves my mouth feeling gross.