Top 5 were:
5 - Dodgy refereeing in 1966
4 - The Shot that Shook JFK
3 - Boxing Day Tsunami
2 - Man on the Moon (or studio floor, depending on your beliefs)
1 - Twin Towers
Actually, being serious it's the first time in ages I've seen the Twin Towers images played again on TV.
Another comment - re: Diana. The image of the crashed car was in the top ten, but personally I feel that it's the images of her funeral that have a greater impact.