[Future Rider Prism]

At first, I was going to say Arsino Magia, but that's a Reiwa monster, and you asked for Heisei, so I went and looked at all the monsters from seasons I've watched, and I've narrowed it down to three monsters. Although, I'm tempted to say, "every monster from OOO or W," but that's defiantly cheating.

First, Burgermon from Ex-Aid. He's just so likeable, you know?

Second, the Orion Zodiarts. Less for the monster itself, and more for the character's story later on. It's a very real story, which I really appreciate.

Thirdly, the Key Roidmude. You'd think the power to open things would be a joke, but this one takes it to Dopant levels pretty quickly.