How do you decide which idea is worth building?

I have a lot ideas on my list, but only one is in active development right now. Part of me wants to build them all at once, but it's not realistic.

How do you prioritize? How do you choose what's worth your focus?

Of course, there are the basic methods, looking at trends, market research, asking friends, asking producthunt... :)

But still, these aren't things you can do in just five minutes. So in the end, how do you decide which idea to explore first?