In my opinion, I think it is Mountain Dew. A powerful marketing muscle has managed to make drinking mountain dew look like a "trendy" thing to do, and now every high school wuss who drinks it thinks he looks cool while he's parked in the burger king parking lot with his fruit colored Honda Civic chugging mountain dew while hitting on the 14 year old freshmen girls. The popularity of the drink has become so strong that even rumors of the yellow-5 chemical it contains effecting your sexual performance have not effected it's popularity. As for the runner up, it's a tie between Red Bull and Sierra Pist(although I am not sure if Sierra is that popular, but I still loathe it due to the blatant fact that pepsi is using it to faze out their lemon lime sprite brand the same way they're using Mt.Dew livewire to kill off Orange Slice).
Be American, buy American.