The Mahavishnu Orchestra at the Paramount NW, Seattle, November 1972. I was completely unprepared for intensity, volume and virtuosity, especially as displayed on John McLaughlin's twin neck guitar. I was on mushrooms. The opening band was Joe Walsh with Barnstorm, which was a guitar fest in itself. I attended a lot of concerts in those days (40 years ago? yikes) but that show was definately in my top 5.
Just a side note. I still listen to Mahavishnu Orchestra concerts from 1972/1973 and I've still never heard a more fluid guitar virtuoso than McLaughlin, I love how he blended Jazz rock and blues into his mind-blowing riffs. But I can't stand anything the man has recorded in the past 30 years. Believe me, I've tried because I have so much respect for him, but his playing these days does absolutely nothing for me. He may have matured as a musician, but it's as if maturity means stripping away all excitement and emotion. Anyone else feel that way?