Overshadowed by bandmates

John Frusciante of Red Hot Chili Peppers. When he left the band to enjoy heroin full time, the song quality significantly deteriorated. They brought him back and went on to make great stuff.

Would that be the music or the heroine?lol...........

I believe Zep had a gentleman's agreement that they would cease to be if any member left the band, breathing or not breathing.

I've been admiring George Harrison's bass playing on "old brown
shoe" of late. 🎸. In an interview he once said that was him
going nuts on bass basically playing it like his guitar. Very cool.

Mapman, I did not know that--it makes up a bit for finding out that some of my favorite Harrison solos were played by McCartney (notably, Drive My Car and Taxman).

What a great topic! Lots of great answers, too. Most of my examples have already been mentioned, the most significant being Johnnie Johnson (Chuck Berry's pianist. Someone interviewed in the movie Hail, Hail, Rock n' Roll, noted that a lot of Chucks songs are in "piano keys", not those written on guitar. Get it?), John Entwistle (you can't tell how good he was from The Who's recordings, but live he was astounding), and Mick Taylor (also in the movie is a scene in which Chuck and Keith are rehearsing "Oh Carol", and Chuck notices Keith is incorrectly playing the guitar figure that precedes a couple sections of the song. He shows Keith that the first note is played with the strings bent up (sharp), the second note played with the strings unbent. Those two notes, back and forth between bent and unbent, in that order, several times. Try as he might, Keith just can not play it! Any 16-year old should be able to, and Keith cites Chuck as his main influence. Lame!).

Being overshadowed varies by degree, and though well known amongst this crowd, there are a couple of guys I can add:

Matthew Fisher of Procol Harum. His organ playing was very much responsible for their original sound (including his infamous J.S. Bach quote in "A Whiter Shade of Pale"), which changed quite a bit after he left (the A Salty Dog album was his last). He subsequently made a few good solo albums on RCA, fairly easy to find on LP for cheap.

James Jamerson of "The Funk Brothers", the Motown house band. Yes, he's well known, but his contribution to not only recordings, but the very playing of electric bass itself is inestimable, and still under-acknowledged (though not by McCartney, who has said that hearing James' playing changed his conception of what bass playing could be). The bass part in Jimmy Ruffin's "What Becomes of the Broken Hearted" (one of the handful of greatest songs ever written, by the way) sends shivers up my spine!