I can think of 2 hypotheses:

1. Different flavors of ice cream melt at the same speed.
or
2. Different flavors of ice cream melt at different speeds.

What a fun science project - Yummy!!
My granddaughter is 13, and she thinks the different flavors will melt at the same speed. She thought you could try different brands. And then I decided you could compare a high fat ice cream and a low fat ["light"] ice cream of the same flavor.

My granddaughter says you need to use the same amount of the 2 ice creams. Maybe you could put some little spoonfuls of each ice cream on a piece of foil on your kitchen counter and see if one kind of ice cream always melted faster.

What if you put chocolate and vanilla ice cream under a heat lamp? That's my favorite idea, but it needs a heat lamp.