Generator Gawl, Kyoushoku Soukou Guyver (2005) Recommendations
Watching Generator Gawl felt like I was watching a secondary story in the Guyver universe. There are a lot of similar idea's and themes, interesting plot twists in both that are pretty cool, some characters are pretty similar (especially Guyver's Mizuki and Gawl's Masami), and of course bio-mecha action with characters that transform. If you like any of the Guyver shows, definitely check out Generator Gawl.
It's the whole Sci-fi aspect of it that brings the two so close together. They have similarities in some of the situations that the characters are placed in, creature designs, as well as a big company being responsible for most, if not, all of the world's problems. Though I did think that Generator Gawl ended better than Guyver, they both still had a wow factor about them. I recommend these to be watched in the English dubbing.
These two anime focus on a male hero that can transform into something that is not human and must defeat an evil secret organization that plans on taking over the world. The Guyver is more violent compare to Generator Gawl, but they do have similar themes of not losing your humanity in the pursuit of justice.