Tokusatsu is a term that is pretty general, as it compasses both kaijus and sentais.
So for kaijus and the Showa era, I would recommend Godzilla (1954), followed by Mothra, War of the Gargantuas, and the rest of Toho’s catalog of films.
For Heisei, I highly recommend the Gamera Trilogy, followed by the Godzillas films that were released before it.
For the Millennium era, check out Godzilla 2000 and GMK. For more recent work, check Godzilla KOTM and Shin Godzilla.