There are plenty of decent additives out there but I will mention 3. BG-44k, Seafoam, Lucas. I wouldnt waste money on the cheaper products either. Read the directions on what you choose but most of these products are used at fill up or work best with at least 10 gallons of fuel or so. Dont expect any chemical to be a miracle cure though, if a problem is suspected it may actually need a real flush or at least multiple treatments.
What I like about Seafoam is the multiple uses the product has. You can use it to clean the fuel system, run it through the intake/ throttle body to clean the intake and valves, run it through the crankcase along with other uses. Seafoam will also stabilize fuel for up to 2 years for storage just like sta-bil, it will also take moisture/water out of the fuel system just like HEET. Do a search on here about Seafoam, plenty of people here have used it with good results.