Which project should be “the one” to work on?

I have multiple start-up projects (early demos) but I am having a problem deciding which project should be the project on which I will spend majority of my time.

I can't work on multiple projects simultaneously so I've made the decision that I'll work on only one project :)

What questions should I ask myself? What criteria should I use to pick the right project? What else could be important in my case?

I need your help! :)