A few years back the answer would have been RC Cola followed by Coca-Cola Classic. I would drink Pepsi if the other two weren't available but begrudgingly so.
At some point I developed a gastric issue related to drinking RC and cut it out completely. Just to be safe I also have abstained from Coca-Cola and Pepsi as well and haven't had any to drink in about 5-6 years now. Sodas that are on the short list of ones I will drink include Root Beer (Mug being my preferred choice), an orange flavored one like Sunkist or Fanta and once in a blue moon I'll drink Mountain Dew though that stuff aggravates my throat.
Surprisingly, my drink of choice anymore is ice water.