I loved Spielban as a kid, and I thought that show had some of the coolest, fastest theme songs. And when I first got back into toku big time, I had one of those compilations of toku OPs, which introduced me to the credit sequences to a lot of the classic shows and their theme songs. I noticed taking a liking to a lot of the ones by Aniki. (Zubat especially.)
For a while, I was majorly into Mizuki's songs, I was quite an Anikihead. At the time, pre-YouTube, there wasn't easy access to a lot of concert footage, so I took what I could get -- I bought Masked Rider Live 2000 at a time before I was really into that franchise just for the songs. (He pwned at that concert.) I had variety shows that focused on Mizuki or had coverage of his legendary 24-hour concert of 1,000 songs. I was so happy to buy Superhero Spirits 2000 to finally see him perform Spielban's OP. (And he had to have enjoyed performing Spielban's songs since he appeared in the show as Spielban's dad, Dr. Ben.) I bought all of Voicelugger just to see Mizuki appear as Voicelugger Gold, and as much as I dislike that show, Mizuki makes that OP kick ass.
That's what I loved about Mizuki so much. He'd take these songs and make them kick ass. He was pretty much the first anisong singer who took the work seriously enough to *feel* the songs he sang. He gave his songs such life and energy in addition to just singing them well. (Zubat's one of my favorites -- sung from the point of view of a vengeance-seeking Zubat, Mizuki adds such anger and sings the hell out of lines about winds of revenge, the fires of grudges and howling hell songs.)
Aniki was dedicated and proud of these songs; he enjoyed singing and he enjoyed performing for his fans. (So much so that he kept pushing himself and kept performing days before his death, when people like Shougo would just be laying around and ranting against his enemies.) In his commemorative song "Happiness," with lyrics written by Mitsuko Horie and Masaaki Endou, he says that singing is his life. His distinctive voice and passion held perfectly the aura of all of the various superhero shows he sung for, making him synonymous with most of these shows for multiple generations of toku and anime fans. His passing leaves a massive (Mazinger-sized) hole in the world of anison/tokuson, which will never sound the same, and those concerts aren't going to be the same without him.