Three pop/r&b nominations:
NRBQ - Yes, they have their devoted grassroots legion, but it's criminal that they never got much mainstream attention. I don't remember EVER hearing an NRBQ song on broadcast radio. And they have to be one of the hardest working bands on the planet.
Francis Dunnery - Apparently had more of a following in the UK from "It Bites" - but he's a bloody great guitarist (touring back-up for Robert Plant and Santana among others), and his solo career essentially went nowhere. You'll hear "My Own Reality" and "Too Much Saturn" on radio occasionally, but all of his albums that I have (Fearless, Tall Blonde Helicopter, and Let's Go Do What Happens) are definitely keepers.
The Pooh Sticks - Deserved to be huge (but weren't).