What mortgage payback period is best?

There are multiple mortgage term lengths (7yr, 15yr, 30yr, etc.). Which is best?

Depends on individual circumstances, but one thing to note is the longer you drag along your loan repayment, the greater the total amount of interest paid over the entire time span on a fully amortizing loan. For interest only loans, since your principal balance doesn’t change and you payoff monthly interest on time, you will pay a standard amount of total interest (less risk of compounding)