I haven't seen many shows (just never really got out much), but I've seen:
Tiesto in 2005 or 2006, I can't remember, at Club Avalon in Hollywood (it was a random 18+ show and I had just turned 18).
Tiesto in 2007, at the LA Sports Arena, which was just epic.
Mindless Self Indulgence in 2007 in Phoenix, for Project Revolution. They were incredible live but it was boiling hot and I wasn't getting along with everyone on that trip.
BT in 2007 on three different occasions (I think one or two were in 2006 actually). I saw him at the Virgin Megastore in Hollywood, then I saw him at Cinespace in Hollywood screening This Binary Universe (he talked with me for a little while), then I saw him perform This Binary Universe live at the House of Blues Anaheim.
Louis XIV last year in Huntington Beach during the US Open of Surfing. They played right on the beach, it was real cool.
Crystal Castles and Metric at the Glass House in Pomona. Great show but I lost my wallet before the show (that place and disappearing wallets...)
Deadmau5 on election day at the Virgin Megastore in Hollywood. My favorite producer at the moment