Mick Jagger
Madonna

I'm looking at it from contributions to great songs. It's not necessarily that they were the "songwriter" but that their guitar playing and arrangements defined the sound.

Lindsey Buckingham is very underrated. He's never on a list of the best, but his guitar arrangements together with his songwriting contributions (often uncredited but he shared other compensations with Mick Fleetwood) define Fleetwood Mac's biggest songs, from "Go Your Own Way," to "Don't Stop" to "Rhiannon" to "Gypsy".

George Harrison was a huge contributor with the Beatles, but nobody takes him seriously as a great guitar player. Particularly when they really started sounding good, as in "Sgt. Pepper's Lonely Heart's Club," Harrison's guitar riffs really rock the songs. You can really see how huge Harrison was creatively with the Beatles on their last arrangements of John Lennon's "Free as a Bird" and "Real Love."

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"Now, bring me that horizon."
-Capt. Jack Sparrow

"Ler" Larry Lalonde original Primus guitarist.

Martin Barre of Jethro Tull and Terry Kath of Chicago are the most underrated guitarists ever in my opinion.