Ritchie might be semi-forgotten now, but I don't think he was ever underrated. Back in the '70s-early '80s, he and Jimmy Page were quite commonly spoken of/written about as the biggest rock guitar heroes in the world. What happened, I think, is that Deep Purple was so closely associated with technically accomplished hard rock and metal that DP and Ritchie were kind of swept out of favor in the early '90s when grunge came in.
Back in the '80s, it seemed like only a few weirdos like me were big Tony Iommi fans (I preferred him to Randy Rhoads at the time), but when grunge came in, all of a sudden all of these newfound Iommi sludge-riff-of-doom fans came out of the woodwork--he and Blackmore almost did a total switcheroo in terms of being influential guitarists. Which is silly, because they are both guitar gods.
Ritchie was one of my earliest guitar influences, and I still find his combination of classicisms, bluesy licks, and reckless abandon very inspiring. A lot of the neoclassical guys could play much faster and cleaner, but they always seem afraid to really take chances with their playing the way Ritchie did for most of his career. I think his playing began to grow a bit lazy around the time of the 1984 DP reunion, personally. But give me any of his pre-reunion DP albums or Rainbow Rising anyday!