Mine is: do others see the same colours that I do? As in, is my "green" the same as your "green"? Or would my "green" look "blue" to you? I like this one because it's completely possible, points out the plasticity of our minds and makes a distinction between sensation and perception. There are variations of this but I like it formulated as such. It's my favorite because it was also my first foray into the philosophy of consciousness and I'm often reminded of it when in an altered state of consciousness (e.g. by psychedelics).
Your favorite experiment can be whatever: either something that has affected you deeply / changed your life or just something fun and amusing to think about.