I've mixed so many sodas that I can't just pick one. I remember the first mixture I ever did was at McDonald's when my brother and I mixed all the drinks together except for the diet soda and the water. We called it a graveyard. They tasted good but if you frank enough of it, you got sick from it.
I do remember one mixture that was gross, I mixed generic lemon lime soda, generic grape soda, and mug root beer together and it tasted like NyQuil, it was awful.
I don't mix soda like I used to when I was younger, but when I do, I like to mix Pepsi and Mountain Dew together.