I got to thinking about that and I'm afraid my own question will be tough for me to answer. I don't go to concerts that often these days, very rarely in fact... so I'm going to have to go back many years.
My "Guess" as to who I saw the most often is: (and note: I'm not including Jazz artists because I don't think many here would know them)
Santana. But that's misleading... I liked Santana, don't get me wrong, but not really at the top of my list of favorite bands. Reason I saw them so much was that in their early days, (already popular however), they played in Fresno a LOT.. and my band opened a few concerts for them, (lot of local groups did this).
Next up I'd say "Elton John". Just guessing here but I saw him many times. Some of those in 'recent' years, most of them in fact.
I saw the Doobie Bros. a lot but like Santana that's because they played here a lot. Many groups I liked a lot more than the Doobie Bros.
Next up I'd guess a tie between Metallica and Iron Maiden... my son liked them a lot, (so did I).
The group I most regret never having seen, (well, I did, more on that in just a moment), is Led Zeppelin. When I was stationed at Ft. Monmouth in New Jersey they played a local concert... friend and I were on duty and couldn't get away until very late.... we rushed to the venue as fast as we could and arrived about half way through their final song.