What is the best flavor of that drink for the bowel prep for colonoscopy?

Your choice. Colon lavage preps are designed to "wash through" the gut without being absorbed. As such, flavors used are restricted to sugar-free products (sugar promotes water absorption), & should avoid colors that stain the gut (red-colored preps are particularly troublesome when searching for bleeding sites). Commercial flavor choices: cherry, orange, wild berry, lemon-lime, pineapple, or no flavor added.
Cherry. Mpst people tell me cherry, but i also tell my patients that they can get the plain and add crystal lite that has no sugar and that way use the flavor they prefer. It is always going to be a little salty. Make it cold and it may be doable.