"Most important" is a pretty broad net. That, for me at least, has as much to do with where your head was at at the time as it does the performer/performance. Top of the list for me would probably be Pink Floyd's The Wall concert at the LA Sports arena in 1980, both because of my state of mind and the impressive sound and staging. A decade or so later, Neil Young and Crazy Horse in the same venue (one of the dates on the Weld album) was pretty damn awesome as well.
Watching Buddy Guy at the King Biscuit Blues festival in Helena Arkansas was fun, but not nearly as mind blowing as those. In the blues category, most of my favorite moments were small club dates with folks like Gatemouth Brown, Sue Foley, Debbie Davies, etc. Most of those I was a few feet away taking pictures with an SLR, not a cell phone-- and sometimes getting paid as well.
Within the last five or so years, I'd say PJ Harvey at First Avenue in Minneapolis was pretty fine, as was Patti Smith at the State Theater. As a parting shot for my time in Minnesota, I really enjoyed the Prairie Home Companion show in St Paul with the Derailers. "Important" is slippery, I would just mostly file these shows under "fun" instead of life or mind altering.
Oh, and I almost forgot-- Tom Waits in Tulsa last year-- a song or two ended up on his latest album. Oh, and one I forgot to mention in the blues concerts was the Fabulous Thunderbirds with Duke Robillard in Bakersfield, California at the fairgrounds. They had to step up at the last minute because SRV and Double Trouble cancelled when SRV died. Jimmy and Duke made for a great wake for that horrible loss.
