My earliest memory would be Ireland beating Italy 1-0 at USA 94
I don't remember much of the match, just the goal and the result
France 98 was the first World Cup where I watched most of it and knew what was going on. My highlights from that were Croatia knocking out Germany and Bergkamp's goal against Argentina
For 2002, England beating Argentina was really enjoyable for me because it was sweet revenge for 1998, and also the perfect story arc for Beckham, especially as Simeone tried to put him off before he took the penalty. Keane's last minute equaliser against Germany was also pretty awesome
For 2006, it'd probably be the final, which was rich with drama. It's probably the World Cup I've watched the least to be honest, as I was working through most of it so missed a lot of matches
For 2010, has to be Germany stuffing Argentina and the fantastic Holland/Uruguay semi final. Both great games. Slovakia knocking out Italy was also an amazing World Cup moment
For 2014, 7-1