What is best to use for a sealer on a fitting in a gasoline tank?

Unless a gasoline fuel system fitting is defective, no sealer is necessary or recommended. IF a joint or fitting leaks, the recommended "fix" is to REPLACE the leaking fitting. IN an emergency [only temporarily], and only until the vehicle can be moved to a repair station, one "might" get away with using silicone plumber's tape, or Permatex [brand name] #2 paste sealer. Although I cannot recall competitor's names, there are products equivalent to Permatex. I recall, back in the 1950s, the "olden" days, that we could use well chewed bubble gum for fuel system leaks, BUT in the 60s, the natural gum base in chewing gums and bubble gums was replaced with artificial, man-made gum which no longer worked because they are dissolved by gasoline.