"Matango: Attack of the Mushroom People",
"Godzilla Vs Hedorah" (Godzilla vs The Smog Monster), & "Godzilla Vs Mothra" ...
Absolutely ~ these are 'must-see's.
Also: What's the one where Goji falls asleep sitting on a rock?
That's totally my favorite Godzilla scene ever,
but damned if I can remember which movie it's from right now ...
And yes, the early "Gamera" movies are def worth a look too.
However, I'm not such a big fan of
henshin or the 'kaiju-wrestling' style myself.
My
favorite TV series is prob "Captain Ultra" (which was very silly indeed)
... but I'd like to give a special mention here to the orig "Ultra-Q" TV show:
"Ultra-Q" was HUMANS vs. monsters, usu. with an underlying eco-message
which Eiji Tsubaraya called 'Unbalance'.
Some episodes are downright creepy: eg the ghost-girl on the railway track.
Some are funny:
M1go wreaking havoc on an express train, also the one with Kanegon.
Others are just simply AWESOME !
The ones with Kemu-seijin and Garamon spring to mind.
Tsubaraya-san also did the special effects for the early Godzilla movies,
including the first one, btw.