Wow! Some great artists listed here. Stanley Turrentine! YES!

Others already listed that I would 2nd, 3rd, 4th & 5th, and throw in a few of my own--mind you, I would have wanted to see them all in their prime, not decline:

Folk/country sorts of acts I never saw, but would have LOVED to see:

Silly Wizard
Sandy Denny
Pentangle
Planxty
Willie Clancy
Ramblin' Jack Elliot
John Prine
Richie Havens
Ricky Scaggs
Earle Scruggs
Dolores Keane
Fairport
The Band
Doc Watson
Patsy Cline
Arlo Guthrie
Woody
Jean Ritchie
Johnny Cash
Roseanne Cash
Allison Kraus
Flyin Burrito Brothers
Emmylou
Poco
Dixie Chicks (I know there is still plenty of hope)
McGarrigle Sisters
Natalie McMaster (again, still hope)

Blues/R & B acts...

Mingus
Coltrane
Stanley Turrentine
Willie Dixon
Lady Blues, Billie Holiday
Bessie Smith
Nina Simone
Wilson Pickett
Sam Cooke
Mitch Ryder & Detroit Wheels
Booker T & the MGs
Boz Scaggs
The Immortal Roy Orbison

Rock/blues acts...

Tina Turner
Ike & Tina Turner
Jerry Lee Lewis
Little Richard
Janis Joplin
The Doors
Hendrix
Rolling Stones
Allman Brothers
Allman Brothers
Allman Brothers
Traffic
Blues Project
Blind Faith
Nirvana
The Clash

Pop music acts...

Sinatra
Tony Bennett
Duke Ellington
Beatles (my mum wouldn't let me go when I had the chance at a free ticket, but then you couldn't hear them anyway, so I don't know it matters much in the end)
Everly Brothers
Marvin Gaye
Shirelles
Martha Reeves and the Vandellas
Hall and Oates (they were kinda like an Everly Bros reincarnation, weren't they?)
Young Rascals (homeboys of my youth)
Joni Mitchell
Van Morrison
Rickie Lee Jones
The Hollies
The Kinks

But really, there are only a handful of acts I truly, truly wish I had seen:

Allman Bros
Janis Joplin
Patsy Cline
Everly Bros
Silly Wizard
Planxty
Mingus
and call me crazy, but Duke Ellington. I saw the Count, I saw Louis Armstrong, but I never saw the Duke. Would have loved to have seen him with my dad, who loved Big Band music and introduced me to it.

And of the greatest singers of all time, Billie Holliday, Nina Simone, Sandy Denny, and Frank Sinatra. Considering how prevalent the bias is towards men in music, I find it ironic that most the "great" singers on my own list are women.