Josh White, an old blues icon, at the Pasadena Civic Auditorium in 1960. The first well-known act I saw was the Kingston Trio at the same venue a few months later. I was in college and folk music was all the rage so I saw the Limeliters, the Weavers, Peter Paul & Mary, Buffy Sainte Marie, etc.
The first bona fide rock and roll concert was Pacific Gas & Electric in a West Hollywood club (near but not on the Sunset Strip) in 1965. My first stadium concert was Creedence Clearwater Revival in 1970. My first insane outdoor hundred-thousand-people-football-stadium concert was the Rolling Stones in 1980 or 81. My first Third World take-over-the-whole-city festival was the Reggae Sunsplash in 1984.
The most memorable was Pink Floyd doing The Wall, from the 16th row. A close second was Aerosmith right after they dried out and went on a small-venue tour to get back in practice, in a place that held about 2,500 people. The spaciest was Tangerine Dream with a laser light show. The most intimate was Linda Ronstadt in a club back when the Eagles were her backup band, sitting close enough to touch my Tucson homie--but didn't. The most moving was Shakira doing the whole evening in Spanish, surrounded by every Colombian in the northeastern USA. The scariest was a Lynyrd Skynyrd survivor reunion, surrounded by every Redneck in Los Angeles. The one I regret missing was Pearl Jam, and the ones I'm so glad I saw several times were the Dead and Zappa. The most awful was Phil Collins, Peter Gabriel was in town and there was a rumor they'd show up at each other's concerts but they didn't.
Most recent was Starsailor, a couple of weeks ago.
Rubik and Genji: The group that I have seen the most times may well be the Blue Oyster Cult. But ironically I loved their first three albums when they were still doing acid rock. I thought "Don't Fear the Reaper" was a sellout to get on AM radio.