I would say that the best atmosphere's i've experienced were the early SL Derby's at The KC, before it got stal, it was bouncing with twenty thousand plus giving it everything. It was so loud you struggled to hear the person next to you.
The New Zealand versus Australia Tri-Nations final at Elland Road somewhere around 2005 ish where everyone turned Kiwi for the night and the Aussie's got humped was pretty good, although I had spent several hours in The Grove before kick off...