my number one choice for writing is Monster energy. Original green, thank you very much. There's a generic energy drink that I don't mind at all.
Most usually though, its coffee. MIlk is a luxury if I have it. Normal would be just sugar though. Real creamer is me spoiling myself.
I get my best results with the above.
I've found that if I have a drink for writing, my MC gets testy and might threaten people. I start to think violence might be called for here. I've learned not to do that. If sober I decide its time for violence in the writing, then a drink or two gets the juices for that flowing.
I will drink hot tea but just for something different and for taste, coffee and energy drinks seem to have an actual effect.
the surgeon general has determined that getting between me and my morning monster/coffee can be hazardous to your health.
any soft drink with caffeine can be substituted for coffee. if caffeine is bad for you, then my kidneys must look like raisins, lol.