I think generally you should pick something where you have that unfair advantage and you like to do. If you are good at sales of course you should pick something where you need lots of sales, same to UI if you are god like designer of course you should pick something where nice UI is advantage.

I agree with this. The harder the thing is you're doing, the bigger your moat. Ideally it shouldn't be too hard for you though of course, e.g. because you have prior knowledge about it.

If you're a designer, pick something that needs to look good

If you're a UX person and have access to a pool of people to do user testing with: pick something that needs good UX

Those two are somewhat at odds with each other, and neither is really the job description of a typical frontend engineer (though of course some frontend engineers are good at one or both)