I don't know what is " best " but I use Sta bil fuel stabilizer.
As an example, I just sold a 20 year old generator that was owned originally by my parents. I acquired it about 10 years ago after my father passed. It would not crank, had old gas in it. I drained the tank, new plug, new gas and got it cranked. Since then been using sta bil in the gas ( same with my boat and other small engines ) and cranking it 2-3 times a year. When I listed it on CL it had not been cranked in several months. Guy called last weekend made an offer and I accepted. He came over and we pulled it out of the back of my storage bin and it cranked with 6-7 pulls.
I am convinced Sta bil works. I am sure there are other products like Seafoam that will do the same, but I have not used them.
When I buy gas for my small engines I go ahead and put Sta bil in the transport tank. That way I always have " fresh " gas in the storage tank and the engines. Works for me.