RTV, Permatex, Aviation Form-A-Gasket, The right stuff etc....

As stated by others, clean surfaces are veeeery important when using silicon. Remove any corrosion, roughen the surface if it is shiny [ chrome, polished alum ] & give it a final wipe with lacquer thinners.
I have used different brands/types of silicon, even cheap hardware store stuff, & have found no real difference in the sealing ability. Around sensors, neutral cure should be used & I only use the expensive high temp sil on exh system gaskets; standard sil is good to 350F+ degrees.