Van Basten's stunning goal in the '88 European championship's final. Being a dutch boy of the age of 12 (at the time) I felt on top of the world and I fear it will be a long time before football will make me feel like that again, if ever..
Close personal second: Feyenoord (my hometown team) winning the Uefa Cup in their own stadium in 2002 just days after the political murder of Fortuin had shocked the nation and Rotterdam where he lived. Most surreal atmosphere ever..
As far as Newcastle is concerned, the best atmosphere and win I can remember witnessing first hand was an FA Cup match vs Chelsea in snowey conditions where Kluivert got the winner with a great header.
God, how I would love for Newcastle to finally win something major. It would be absolutely amazing in a city so footballminded as Newcastle!