Yes, there was a gap between their tenures, but Jerry Donahue was essentially Richard Thompson's replacement in Fairport Convention. That right there says plenty about the caliber of player Jerry Donahue was.
Someone mentioned Andy Gill (Gang Of Four) already (thank you!!) ... I concur with Mick Grabham (Procol Harum, others); the best guitarist PH ever had. Peter Tolson, the Pretty Things. From Freeway Madness to Cross Talk. Also played with xPT's on their reimagining of Parachute. As good as anyone in the whole rock'n'roll circus, in my humble opinion. I may be biased, I toured with him briefly.
Denis D'amour (Voivod) Doug Rodrigues (Music, Mandrill, Santana, Terry Reid...) Brian Godding (Blossom Toes, Magma, Mirage, Mike Westbrook, Kevin Coyne...) Jamie West-Oram (The Fixx) Paul Fox (The Ruts)
I believe Kraan's Peter Wolbrandt was also an influence to Ed (check from 04:50 onwards). Another underrated player.
Ernie Isley. Dropped quite a few rock solos in the middle of Isley Bros. R&B ballads and/or covers of rock songs, such as Summer Breeze.
More under recognized than under rated but Richie Kotzen. Best guitar player I have ever seen in my opinion. Not a style of music that dude doesn’t play
Robby Hoddinott (Kingfish) George Kooymans (Golden Earring) J. J. Cale Danny Kirwan Byrd Burton (Amazing Rhythm Aces) Steuart Smith (Eagles)