Top of the list for me has to be seeing the Who around 1970-71; a recently deceased family friend happened to be in Leeds on the evening ofFeb 13th 1970...
Led Zeppelin about the same time would be another band I'd love to have seen live, or Space Ritual era Hawkwind.
A little futher back, and oh to have been in the Grande Ballroom in Detroit on 31 October 1968 as the MC5 kicked out the jams.
Moving back in time, seeing the original line-up of the Skatalites with Don Drummond would have been awesome.
Back into the 1970s, Van Der Graaf Generator about 1975 is something I'd love to have seen but I did get to see the classic line-up on the reformation tour in 2005. Would also have loved to see Thin Lizzy in their prime before the drink and drugs took too much of a toll. AC/DC with Bon Scott likewise.
From the punk era, seeing the Ruts would be top of the list though experiencing Throbbing Gristle in the flesh would be another good one.
The classic Motorhead line-up - I was always jealous of my older sister's ex boyfriend who was at Port Vale festival in 1981 and saw not only Motorhead at their peak, but Ozzy Osbourne with Randy Rhoads on lead guitar! I saw them in 1990, pretty sure that was the loudest gig I've ever seen but still would love to have seen them in the Fast Eddie days.
On a quieter front, would liked to have seen Nic Jones before the devastating car crash that ended his career.
Moving on into the era when I was going to gigs, I'm still kicking myself for not going to see Mudhoney or the Neph. I did see Nirvana twice but I didn't rate them as a live band, it felt like watching a band practive. Pearl Jam were a damn sight better live...
Likewise, I really should have gone to see David "Honeyboy" Edwards when I had the chance - far too late now!
On the other hand, for things I did get to see that retain their "wow" for me... Naz Nomad and the Nightmares (the Damned's alter ego band) at the Leadmill in Sheffield, the Pixies secret gig at Scarborough Futurist in summer 1991 (they'd been rehearsing there prior to their European tour and did a warm-up), NMA's low-key gig at Buckley Tivoli in 1996 (last time I saw them with Rob

), the Levellers with the Moonflowers supporting, Skyclad on their first tour. Happy days...