Carlos Alomar.
He collaborated with a lot of solo musicians, so he wasn't a "band member "and probably less known in general. Most famously, he was on a number of Bowie's best albums. Very distinctive style.
Here he is with Iggy Pop and David Bowie.
If we're stretching the use of "Unknown," which people already have, I have to say Frank Zappa. Name might be known, but he never had a top 10 single or album( I don't think) and isn't a regular on any classic rock stations I know.
While he did make some 3 minute pop rock songs, he also made a lot of experimental, Jazz influenced 15 minute songs, which aren't going to get mainstream play. And even composed classical music with orchestras. When people talk of best guitarists ever, and they name the usual dime a dozen with their usual bag of tricks, it's a shame.
Here is a more "Standard" rock composition, with Captain Beefheart.
Here is a beautiful Jazz Fusion one.( Zappa composed an arranged this all himself...has about 20 musicians)
...oops that is the full album. Oh well. Song I meant was the first one anyway, and the whole album is great.
Here are two of his own favorite guitar pieces...