If the question allows for past groups I would have to say Zep. If that isn?t allowed I would go with the Black Keys because I haven?t seen them.
Seen Elton and Joel....much bigger fan of Elton but Joel stole the show that night. Would definitely see either again..
Saw this combo at Wrigley a few years back, it was enjoyable but it paled to seeing Joel the year prior at the Last Play at Shea. Part of that was the myriad of guest performers he had though, Tony Bennett, Garth Brooks, Steven Tyler, Roger Daltrey and capped by Paul McCartney.
Other great shows were seeing No Doubt when they were first starting and Weezer opened for them. (ND was so much better at the beginning when they showed their punk/ska influences.) Also Rush puts on an amazing show and saw OK Go at an uber-small venue when they were first starting out.
Best part about those, in my opinion, is that I would consider none of those to be one of my favorite bands, its just that the live performance was so great.