Whichever problem you have that you would pay the most to solve.
If it isn't a problem that you actually have, then it probably isn't worth solving. If it isn't a problem you actually have, it's likely going to be much harder to solve (even if it exists).
If it's a real problem that you have, it's likely that somebody else has the problem too. After you identify the problem, it should be easy enough to ask around and see who else has the same problem.
If it isn't a problem that you actually have, then it probably isn't worth solving. If it isn't a problem you actually have, it's likely going to be much harder to solve (even if it exists).
If it's a real problem that you have, it's likely that somebody else has the problem too. After you identify the problem, it should be easy enough to ask around and see who else has the same problem.