The short answer is that almost any cold drink works, but some come out much better than others. After enough tries you start to notice a pattern, and it comes down to what is dissolved in the liquid.
What works best
Soda is the easiest place to start. Cola, lemon lime, orange, root beer, all of them crystallise cleanly. One tip that makes a real difference: let the can sit open for a few minutes first. A freshly poured soda foams up when it hits the cold wall, and foam is air, not liquid. Less foam means more of your drink actually touching the surface that does the work.
Sports drinks and sweetened iced teas are the other reliable group. They are mostly water and sugar, which is close to ideal. Blue raspberry and fruit punch give you the flavour people remember from a gas station machine.
Juice works but behaves differently. It has more body than soda, so it comes out thicker and takes closer to the full sixty seconds. Do not rush it.
What to be careful with
Anything with pulp or solids in it. Fresh pressed juice, smoothies, drinks with fruit pieces. Solids get in the way of the liquid moving against the cold wall, and you end up with something half frozen and uneven. If you want to use fresh juice, strain it first through a sieve.
Dairy is a grey area. A splash of milk in a coffee slush is fine. A drink that is mostly milk turns into something closer to a milkshake, which is not bad, just not a slushy.
Anything warm. This is the one real rule. The cup holds a fixed amount of cold, and a warm drink spends all of it before it can freeze anything. Chill your drink first, every time.
A note on sugar
Sugar lowers the freezing point, which is why a sugary drink turns into smooth slush while plain water turns into a solid lump. If you are using something unsweetened, expect a coarser, icier texture. That is normal and not a fault with the cup.
