My brother has always sworn by Yamaha oil, because it is the exact same brand as the motor that he uses on his boat. He trusts oil that is made by the same manufacturer, because they are bound to know their product better than any other company. Using the oil also keeps his outboard motor under warranty, which is important.
The only problem with Yamaha oil is that it can be sort of expensive. Over time, my brother has learned not to buy the oil from the marina, where prices are almost always high. Instead, he buys all of his oil from online sites. The oil costs a lot less. He is able to actually afford quality oil because of this.