The cheapest term life policy is the one with the lowest price for the term or duration of your coverage.
So, the shorter the term, the lower the price. This means, a one year annually renewable term life insurance policy would offer the cheapest coverage for a one year period of time.
However, each year you choose to renew your policy the premium will increase, so over time it may end up costing more per year on average than a 5 or 10 year term policy depending on how long you need the life insurance.
That's why it may be most cost effective to buy a 5, 10, 15, 20 or 30 year term life policy since your premium is guaranteed never to increase during the term of your term life plan.
It really depends on how long you need your life insurance protection to last.
Once you determine the right length of time you need coverage you'll want to compare term life insurance rates among the leading insurers to find the most affordable price.