the best concert i've been too would probably be the Rolling Stones one, and the Travis / Coldplay one. I'd KILL to see U2 live. I hear their recent shows have been killer, but oh well. Logistics and stuff :S
Watched Bon Jovi some years back too (hangs head and shuffles feet) but that was in their prime and it was a good show.
but the most technical impressive was Mr Big. Nabe|, I remember, Paul Gilbert picking the strings furiously, shredding speed, and Eric Martin helping him fret from standing behind him! And Billy Sheehan putting the bass on a small table and playing it like a keyboard. Damn cool