I am interested in buying commercial real estate for income. What type of properties have the best return with the lowest risk? Retail, office, industrial, or multi-family?

The answer is all of them and none of them. Not to get cute, but the return vs. risk equation can really be the same for each of those property types. Your return is not determined by the property type but more by the quality of the property, reliability of the tenant, ease of re-leasing the space, cost of maintenance and likelihood of appreciation. In other words, you can make a great return from a retail, office, industrial, or multi-family property or you can get whacked by any of them. The trick is to buy wisely and weigh those factors carefully. The more successfully you can do that the better your return.