Start with Now: problem/solution fit. Your goal should be to learn if what you are working is a go or no go
Identify and tackle risk as soon as possible. Identify what's riskiest and start with that. Not with what's safe or easiest.
The "supply" side of a product is a given most of the time, i.e. it can be build. It's the "demand" side where the risk lies. Do customers want this? Are they going to pay for it?