What is the best fuel system cleaner?

Well, Seafoam is quite effective for cleaning out the whole system, as well as your stabilizing your gas, because it has a high alcohol content. One problem with this is that a lot of older engines aren't made to be ran on a whole lot of alcohol. Before ethanol gas, putting in a bottle of seafoam didn't make too much difference, and the engine could handle it well, but now mixing ethanol along with seafoam, can possibly cause issues with your engines down the line, BUT it will clean your injectors and gas :-) I recommend Chevron's Techron system cleaner. It will clean your injectors, stabilize your gas, and clean your fuel level sensor, if its in your tank, As an added bonus its petroleum based rather than alcohol based, so its less harsh on your engine.