In general, it's best if you're building something that you yourself need.
No matter what you choose, build stuff and be around smart people.
In general don't start a startup you're not willing to work on for ten years.
Because so few people make an actual long term commitment to what they're building, the ones that do have a huge advantage.
A board member of mine used to say sales fix everything in a startup, and that is really true.
It takes years and years, usually a decade, to create a great startup.