There is not a single best sealant for guttering since there are so many products to choose from and what you select depends on the gutter system itself.
Typically speaking though, the leading brands tend to be Geocel, Evo-stik, Everbuild, Soudal, Unibond and a whole host of specialist sealant for guttering and roofs products.
The sealant you select will depend on the material of your guttering. At PIRS we stock an extensive range of gutter and roof sealants from high grade mastics & silicone to fire rated expanding foam, butyl mastics and more.