Gamera and Gyaos in general are iconic. Heisei Gamera, Gyaos, Legion and Iris are some of the best kaiju designs period.
Barugon is a great lizard-y inspired design that contrasts well with the turtle-y Gamera. Virus and Zigra are great, sleek deep-sea inspired alien designs (because so much about the deep-sea is alien to us). Whacky Guiron and fugly Jiger are great 70s era kaiju designs.
The underrated Gamera the Brave features great 'back to basics' designs with the younger, inexperienced Toto and the Barugon-inspired Zedus.
The 50th anniversary short is worth noting too, which features pretty cool Gamera 3/Legendary Godzilla inspired designs.
Exterminate all rational thought.