The Sparks actually have a little cult following in my town, which is how I was exposed to them. Bear in mind that this cult following is limited to a few apartments scattered throughout town, but those involved are rabid advocates of the band. And so considering that I'm pretty well-read on the subject of popular music and had never heard of them before moving here, I'd say they're pretty underrated.
I'll throw a vote in for The Kinks, who are easily one of my favorite bands and, as far as British invasion stuff goes, were easily the equals of the Beatles and Rolling Stones and SO much better than The Who. Their early singles are brilliant and Ray Davies developed into one of the great songwriters of all time around Face to Face. And while they're acknowledged from time to time (particularly in relation to Blur, who they obviously had a major influence on), they're really not as revered as they should be.
