These are definites for me, I'd love to have seen them in their pomp:

• The Clash
• Thin Lizzy
• Ian Dury & The Blockheads
• Queen
• Dio (or even Dio-era Black Sabbath)

If I started listing them now, the work day would be over and I'd still be typing... 

 
I know what you mean Craig, I could have added a lot more!

Queen (anytime)

Elvis (early 70's)

Judas Priest (every tour between Hell Bent and Defenders) 

Bowie and the Spiders

The Beatles

Led Zeppelin

Black Sabbath (original line up and with Dio)
 
KISS in 1977 or before
 
Ozzy with Randy Rhoads

Hendrix (New Year's Eve 69)
 
Iron Maiden (Killers era)

Tool
 
Cradle of Filth
 
 
 
 
J.S. Bach, chapel of Duke Johann Ernst III in Weimar era. ;)
 
 
 

Hendrix.
He's really the only one.
 
All the rest?  A blu-ray works better for me. No traveling. No people I don't care to be around. I can enjoy more than once. Just more value in my time and money than dealing with the public while trying to enjoy a musical performance. That opinion was made many, many years ago.

Rain
 
Ozzy with Randy Rhoads

Yeah, good call Krist.

The Beatles (even though I've seen Macca and Ringo separately)
Cream (even though I saw Clapton and Bruce  separately)
Queen
Pink Floyd
 
Those are the top four. All else matters not.

Joe Bonamassa (never going to happen, his ticket prices are through the roof)
 
Graham Parker & The Rumour
 
Kate Bush
 
Streetwalkers
 
Gentle Giant
 
Rory Gallagher
 
Gary Moore

Johnny Cash
The Doors
early The Who with Moonie in his prime
Benny Goodman ca. 1938
Jazz at The Philharmonic, September 1952

Definately Thin Lizzy, see avatar.

Gary Moore
ZZ Top (missed them at Blues Passions)
John Fogerty (still dreaming)
Horslips (maybe ...... one of these days)
David Gilmour
And many others......

Cheers
Jerry