95's Gamera, Guardian of the Universe is a good watch. The sequels are a mixed bag, with technically better effects and some genuinely interesting things going on, but I have issues with both of them that I don't with the 95 movie.
1998 Godzilla feels like it was written as an update for "The Beast of 20,000 Fathoms", for which it would have been more suitable, but got shoehorned into a Godzilla film instead. It's a serviceable movie in its own right, and the animated spinoff that followed kind of makes up for its mistakes, but its not a great Godzilla film, no.
"Frankenstein Conquers the World" is probably worth a watch as an example of classic, non-Godzilla Toho (though co-star Baragon got folded into the Godzilla mythos later).
"Daimajin" shares DNA with Kaiju films, though I would call it mostly a samurai film with a kaiju-esque final act. That final act, though, is worth the wait.