Nothing is created in a vacuum and I felt like exploring what inspired our beloved franchise.
I'll leave any connections to Kamen Rider to those who have a better understanding of that series.
Baoh
This is one that the vast majority of the forum will already know but it seems only fair at least give it a passing mention. For those new members Baoh (Boah: The Visitor to give it it's full title) was created by Hirohiko Araki, best known for his series Jojo's Bizarre Adventure. The basic plot is about a young man who is infected with a bio-weapon that grants him superhuman abilites which he uses to fight against the evil origination that experimented on him. It comes off a proto-type version of Guyver in many way although aspects of Jojo can also be seen here such a beings with psychic power and the Piller Men, especially Kars seem to be based on Baoh.
Mazinger Z
As strange as it sounds Mazinger's dna is in Guyver. Although it's not as overt as stuff like Baoh and Kamen Rider.
While the original show's narrative is very simple it's basic elements are not to dissimilar and more developed version such as Shin Mazinger have a much stronger comparison. There also Mazinger Z's weapon. The most obvious is the Mega-Smasher being in the exact same location as the Breast Fire.
However comparisons could be made with most of them such as the High Fequancy Blade and the Iron Cutter (see above)
Head Beam - Photon Beam - both are the weakest attacks of each.
The Sonic Buster and the Rust Hurricane are a bit of a loose connection however the being Mazinger was based on, Demon Lord Dante, did posses an ultrasonic roar attack.
Another connection would be that fact that in both cases, these weapons are horribly overpowered and make short work of most of their opponents. Which tends to force the bad guys to come up with strategies to put the hero at a disadvantage in most confrontations.
Another is actually Guyver's own Self-Defence Mode. Upon first piloting, Kouji had no idea what he was doing so Mazinger Z went on a rampage upon activation.
A similur event occur during the Mazinkaiser series, the Mecha's power was so great that Kouji passed out and Mazinkaiser entered it's "Demon Mode" and attacked friend and foe alike.
Another possible connection is between Great Mazinger and Guyver II. Great Mazinger was built to be 10x more powerful then Z and it's pilot was built up as a trained specialist. Yet Great Mazinger regularly gets completely trashed in most of it's fight and out performed by Mazinger Z when the two are together. However the contrast is that Lisker was lumbered with a damaged weapon while Great Mazinger is proof just training and natural talent is no substitute for experience and battle honed skills.