So many highlights (many which have already been mentioned). Personal favourites: Ben Johnson beating Carl Lewis and everyone else in 1988 (before we knew about the drugs) was incredibly exciting, as was Usain Bolt's 100m gold in Beijing.
Also, the final lap of Tergat vs. Gebresallaise in the 10,000m in 2000.
However, to pick just one I second Sussex Runner. Ovett winning the 800m in Moscow, 1980. The build up to that race was huge (maybe the only time an athletics rivalry has been the biggest sporting story for months and months). As a massive Ovett fan, it was fantastic to see him "boss" the 800m final and win gold. Ironically, before the Games the majority view was that he would win the 1500 (having been pretty much invicible over that distance for the past 3 years) but wouldn't win the 80m, which was seen as his weaker event.
The only disappointment was he didn't go on and win the 1500 as well and arguably he was never quite the same force again.
Away from athletics, I always find the weightlifting fascinating and watching the Iranian super-heavyweight gold medal winner lifting an unimaginable weight in 2000 (or was it 2004?) was another highlight.
