Diet Coke - I search for wherever it is on sale each week. I prefer cans but will break down and get a 2 liter if that is the only thing on sale.
I will occasionally drink Diet Dr. Pepper, but I prefer the Cherry Vanilla Diet variety.
My favorite is Diet Coke with Lime, DH's favorite is the new Black Cherry Vanilla Diet Coke.
I WILL not drink Diet Pepsi - it tastes horrible to me!