You know, I've seen this asked a lot.
And oddly, it seems to reach the rather interesting conclusion most of the time that the majority of people in the US prefer Pepsi, while the majority of people in the rest of the world prefer Coke.
You might think that's just random, but there might be an actual reason for it. Apparently Coke sold in the US is made using High Fructose corn syrup.
Just about everywhere else, it's made using cane sugar.
Myself, I prefer Coke. Although over recent years I've developed a taste for Cherry Coke instead.
Pepsi is OK, but to me it tastes like every generic cola brand there is. And most of those are much, much cheaper.
As for 'diet'... Eh, diet anything tastes bad enough to me that I'd rather have water. Which is annoying because they'll often sneak those awful artificial sweeteners into cordial, but not make it obvious from the packaging.
Now why would I mix something that tastes that awful into water, if I'd actually just prefer plain water given the choice? Defeats the whole purpose of drinking stuff like that if it doesn't even taste good.