First, a mea culpa on my part. Some of my above nominations are completely inappropriate in terms of the thread’s stated topic and intent, some of them noted by others as being sidemen who later became "leading men" (using the analogy of actors). I will also concede that most of my choices had not been in "monster" bands, but rather in cult-level ones. I don’t consider that distinction artistically relevant, but this is not my thread ;-) .
But there is one name that simply can not be ignored: Richard Thompson. While his output in Fairport Convention may be the equal of his solo work in terms of quality (imo an unnecessary comparison), his solo body of work is far more extensive in quantity. Live on stage Richard is absolutely, unbelievably great! See him before he dies :-( .
And then there is David Lindley. Though he made his name playing on the albums of many of the L.A. singers/songwriters, Lindley was before that in the L.A. band Kaleidoscope. I much prefer his solo albums, and live he is astounding! Sharp dresser, too ;-) .