I would choose the one you can't stop thinking about. Or the one that many of the other side projects eventually lead to. If you feel similarly about all 100 side projects, probably none of them are worth doubling down on.

This is the paradox of choice. In most cases, it doesn't pay to spend your time hemming and hawing about find the perfect project. It helps to just pick one, even if it's the stupidest idea of the lot.

The alternative is to wind up as Burdian's Ass, who is equally hungry and thirsty, can't decide whether to eat hay or drink water, and so dies of hunger and thirst.